If they're used to mechanically heavy systems they'll probably take to Fate Core just fine. Conversion from Fate Accelerated → Fate Core is also easy peasy; the challenge of the conversion is the choices that need to be made in the conversion process.
Briefly: diving into Fate Accelerated vs Fate Core
If your players are going to get tripped up on Fate, they're going to get tripped up on the stuff that Fate Core & Accelerated both have in common: the Fate economy and the game's narrative-first focus. (For gamers coming from a D&D background, it's my experience that there's also a proclivity to resort to physical combat to push the story forward, because it is the go-to means of advancing the story in D&D — but it isn't in Fate.) This means you're going to have to address those issues in the beginning regardless of which version you choose to start with.
(If it helps make it clear: Fate Accelerated is just Fate Core with some bits swapped out and some other bits made simpler, so as to facilitate fast pick-up-and-go.)
However, Fate Accelerated can work well as a sort of palate cleanser for people coming from mechanically-heavy game backgrounds. The lack of complex mechanics pushes the game's narrative focus and the Fate economy more into focus as a means of getting things done, which can be pretty helpful for showing those to your game group.
Approaches particularly will help you push narrative first. When you use an Approach to do something, it's important to narrate what you're actually doing and how you're doing it, and then you determine the Approach to use based on that narration:
Alex: “I forcefully open the door.”
GM: “How do you do that?”
Alex: “With these lockpicking tools.”
GM: “That sounds like it's Careful, not Forceful.”
Alex: “But I only have Careful at +1! Can't I use Forceful? It's my +3 approach.”
GM: “Only if you can find a Forceful way to open the door.”
Alex: “Ok, I'll try to kick it in.”
GM: “The door's made of hard oak wood, so that'll be very hard: roll Forceful against a difficulty 4 to see how you manage. Smashing this door open is going to be pretty loud, too. The guards you saw earlier will probably overhear you doing it.”
(This could lead to a few different decisions. Alex could decide to try picking the lock anyway — it might be easier — or decide to find a different way around this particular obstacle. Alternately the group might decide the the guards finding them would be fun and exciting — that would be an Event Compel, and they'd get fate points for accepting it.)
Converting from Fate Accelerated to Fate Core
This is going to be straightforward. It's going to take a lot of thinking, and the feel of your characters will change, but I'll get back to that in the next section.
Fate Core & Accelerated character sheets only have four chunks to them: aspects, skills/approaches, stunts, and stress & consequences.
- Aspects: Keep them for your new Fate Core character. Feel free to reword them during the conversion process. As you pick up new Skills instead of Approaches, you might find certain details in your Aspects are now unnecessary to say, or that you need to add some characterisation to your Aspects that Approaches were doing for you before. You may also want to do some rewording to assist you in picking up the Stunts you may choose.
- Skills/approaches: Drop your Approaches entirely. Pick Skills as you would for this character if you were making them fresh. Don't focus on trying to equate approaches = any set of skills, because there isn't a strong correlation. Instead think about who your character is as a person: were they powerfully built? (Is that why they were forceful?) Maybe they'll have high Physique.
- Stunts: Replace any of them that you want with the more complex Fate Core stunts. You may need to do this to recreate various capabilities you want your character to maintain.
- Stress and Consequences: Recalculate your stress & consequence slots based on your skills. You'll now have two stress tracks (Physical and Mental) and skills like Physique and Will will modify how many slots you have in each.
You're basically making a fresh new Fate Core character inspired by this character you have, re-using the aspects and stunts they had insofar as you find they're still helpful for expressing who the character is in Fate Core.
If you introduced any Extras to the Fate Accelerated game, convert them using the same process.
The challenge: Different character feel.
Fundamentally, Accelerated and Core characters work differently because of how approaches and skills differ.
Approaches make characters operate with a sort of fuzzy TV Logic. They don't care about your characters' actual skills so much as what kind of person they are and how they approach situations, and how strongly the thing they're trying to do corresponds to the kind of person they are. You'll do better or worse at actions depending on whether they're Your General Sort Of Thing.
A popular example would be Stargate: you have a couple of sciencey characters. They're really Clever and Careful. They have an implausible number of PhDs and areas of expertise for their age. Anything that is a Clever or Careful thing to do, those characters are probably the ones doing it. Then you have the Wisecracking Leader, who's Flashy and Sneaky. Anything Flashy or Sneaky, he'll be the one doing it. Then there's the Forceful and Quick guy: when the show needs someone to pull off feats of incredible physique or athleticism, he'll be the one to do it.
To do something a person still needs justification granted by their character aspects (or other aspects), so Teal'c, the Forceful guy, still has his limits as to how much he can lift or break, but in general capabilities are going to be pretty broad.
Skills on the other hand more closely match to real-life competencies: you have a limited number of things you're good at. Competency is less about what kind of person your character is, more about actually having the skill that says you have expertise at doing that specific thing.
Switching over from TV Logic to a skill system that matches real-world competencies more closely is going to be a change in how your characters feel to play and what they'll be competent at. This change will be felt more strongly the more heavily you were utilising the TV Logic nature of approaches.
So whilst conversion is straightforward enough, your players will need to put some thought into how they want to re-represent their characters in that new paradigm.
Skills aren't better or worse than approaches — both systems are awesome, and both have their time and place for different kinds of stories. When you convert, you might find you preferred Approaches instead because it better suits your characters and story, or you might find Skills were what you needed all along.