[RPG] Can you retrain a feat acting as a prerequisite if you get a different ability that fulfills the prerequisite

This looks to be entirely up to DM discretion.

There are two equally valid positions on this and the rules do not refute either one.

The first is that this is a completely separate spell, and slot, and that the two have no overlap with your casting class. The spell you learned and the slot you obtain are kept separate from the class spells if you have levels in that class.

The second position is that it is a class spell, from your class list and you can retrain it as you would when you take a new level in that class.

The second position is mildly problematic in that you can continue to obtain higher level spells with the retraining, and it may lead to this MC being simply the obtaining of another 1st level spell slot, rather than feeling like a mini MC.

The first position is problematic in that it introducing additional bookkeeping and may be perceived to be overly harsh and pedantic.

In my games, I think I would allow the MC feat to simply be an additional first level spell slot and allow the spell to be retrained if it's from a class that allows that and you take levels in that class. I would probably not make them remember which spell goes with their MC beyond that the extra slot must be a spell from that class. This may be too lenient for some games though.

While a character that first takes one or more levels in the class gunslinger (or another class that gains the character the class feature grit) is prohibited from taking the feat Amateur Gunslinger, such a character is not prohibited from taking the rogue talent grit. The character won't meet the prerequisite for the feat Amateur Gunslinger that comes from the rogue talent grit, but the character still gets the additional grit feat from the rogue talent.

Note that such a character would still have the feat Amateur Gunslinger, but the character won't realize any of that feat's benefits, and because the feat's source allows no other choices beyond that feat, the feat Amateur Gunslinger isn't eligible for retraining.

If, still later, such character gained another level in a class that granted the class feature grit (like an additional level of gunslinger), the character could immediately trade the feat Amateur Gunslinger for the feat Extra Grit. (This is strictly according to the rules as written. A kind GM may allow a character who already has the class feature grit who takes the rogue talent grit to immediately swap out the feat Amateur Gunslinger for the feat Extra Grit without mandating the character to take another level of, for example, gunslinger.)