Would it be game breaking to allow a Warlock to use his Mystic Arcanum to cast his spells at a higher level?

Question

I'm looking for ways to make the Warlock feel more flexible with his castings, and I like this solution, especially since it's limited by how few spellslots Warlocks have of higher level spells (essentially 1 for 6th level+).

Here's my proposed stipulations:

• The Warlock can choose any spell he currently knows to cast at the level of Mystic Arcanum that he currently possesses (e.g. Cloud of Daggers, using his 6th level Mystic Arcanum.)
• That Mystic Arcanum usage could only be regained upon long rests, a la RAW.
• The Warlock would still only choose one spell per Mystic Arcanum that they learned.
• The Warlock would also be able to 'upcast' his lower level Mystic Arcanums, should they allow it (e.g. Create Undead, at level 9).

Would this break the Warlock class in some way or have an overall negative impact if I allowed this?

For further clarity, the player has a Fiend Patron with a Pact of the Chain boon.

Answer 1

Would it "Break the Game"? Probably not

With only rare exceptions, lower-level spells that are upcast as higher level spells are weaker than spells native to the upcasted level. Giving a warlock the ability to upcast their regular spells (or other Mystic Arcanum spells) gives them some modestly improved flexibility, but it's pretty unlikely they'll be able to use it for some kind of absurd cheese, especially given that Warlocks already don't have a terribly robust list of spells. At the very least, it's unlikely a Warlock with this ability would be able to abuse a cheesy strategy worse than an equivalently leveled Sorcerer.

I am, however, of the opinion that it's probably not a good change. I don't know that balance considerations factor in so much as flavor considerations. Mystic Arcanum spells aren't just individual spells you have to cast like a 3.5ed!Wizard, but special spells granted to you by your patron. That is an important flavor difference between Warlocks and other casting classes, and I personally feel that it's important to preserve that flavor.

But like I said, mechanically, there's no real problem. This change makes Warlocks modestly more powerful, but they're not going to break the power level ceiling already established by 5e!Wizards from this change alone.

Answer 2

The real effect is on spell selection

As it stands, the warlock is strongly incentivized to select Mystic Arcana that they can use basically every day, rather than more situational spells, because any spell they can't use for its intended purpose on a given day is basically wasted for that day. With this change, that goes away almost entirely. They're near-guaranteed to have a general purpose spell or two in their standard spell list.

Overall effect will be that they won't be selecting use-every-day spells, which means that they won't be using that small set of unique powers once per day, and they might choose a spell or two that they never use other than downtime - thus making their spellcasting-in-combat profile that much more monotonous.

Warlocks don't really need any more "monotonous" in their spellcasting-in-combat profile, they don't really need the power-up (assuming you have a decent number of encounters per day), and it carves away a bit of their uniqueness. It's not a major change, and it won't break everything, but the overall effect seems like a net negative.