How does the Transmuted Spell metamagic work with Flames of Phlegethos?

Question

In Tasha's Cauldron of Everything there's a new meta magic option that reads

Transmuted Spell
When you cast a spell that deals a type of damage from the following list, you can spend 1 sorcery point to change that damage type to one of the other listed types: acid, cold, fire, lightning, poison, thunder.

and I'm wondering how this would interact with the secondary part of Flames of Phlegethos

Whenever you cast a spell that deals fire damage, you can cause flames to wreathe you until the end of your next turn. The flames don’t harm you or your possessions, and they shed bright light out to 30 feet and dim light for an additional 30 feet. While the flames are present, any creature within 5 feet of you that hits you with a melee attack takes 1d4 fire damage.

If I cast green flame blade, a spell that deals fire damage, then transmute the type to say acid, would the flames still trigger? If not would that mean that casting Booming Blade and changing its type to fire would trigger the flames?

For clarity, the part I'm unsure on is that they both say "when(ever) you cast a spell...", which doesn't really offer any clarity on which would actually take effect first.

Answer 1

Ask the GM; though Xanathar's provides some optional guidance on simultaneous events

Both of these features happen "When you cast a spell" thus, they happen at the same time. Xanathar's Guide to Everything has a section on this sort of scenario:

[...] If two or more things happen at the same time on a character or monster's turn, the person at the game table-whether player or DM-who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character's turn, the player decides which of the two effects happens first. [...]

Thus, if your GM is using this optional rule and they decide to apply it in this instance; it would (in most cases) be the player's decision to determine the order in which these features are activated - there are two possibilities:

1. The Metamagic happens first, changing the spell's damage type and making Flames of Phlegethos either no longer work (if changed from fire damage) or making it now work (if changed to fire damage)

2. The Flames of Phlegethos happens first, granting all of its benefits and then the spell's damage type can be changed afterwards to something besides fire.

Whether either of these actually happens will be up to the GM and whether/how they want to apply the optional rule from Xanathar's. At the end of the day, these features are triggered by the same event - the casting of a spell - and how simultaneous events are resolved is squarely in the GM's wheelhouse.