Arcane collar is not applied when taken, only when manually dismiss and resummon

[taltamir]

The talent arcane collar is not applied immediately when taken. Rather, you need to manually dismiss and re-summon all your summons for it to apply.

While it is true that summons can have different costs based on charge level, said charge level is kept track of and as such it is entirely possible to recalculate MP when arcane collar is leveled up.