Grit, swashbuckler, rogue talent, requirements.

[Seppo87]

I have 1 swashbuckler level and I decide to advance as a Rogue thereafter.

I get the Grit rogue talent.

This gives me a grit feat of which I need to meet requirements AND the amateur gunslinger feat.

Amateur Gunslinger states that you can't get it if you have levels in a class that grants the Grit Class Feature.
I do not have levels in a class that grants Grit but I have levels in a class that grants Panache.

Panache and Grit stack and iirc they count as the same thing for meeting requirements.

Amateur Gunslinger also becomes Extra Grit if I LATER take levels in a class with Grit. It doesn't mention what happens if you have that class already except that you don't meet requirements for the feat if you have the Grit class feature.

What do I get?

a) Only the Grit Feat because I do not meet the requirements for Amateur Gunslinger, and it cannot be switched with Extra Grit because it only happens if I later get classes with Grit
b) Amateur Gunslinger works as normal because while I have to meet the Grit Feat requirements, the same is not stated for Amateur Gunslinger, so I bypass the "not having levels in classes that grant grit"
c) Same as B, but the reason is different: The "grit class feature" is a specific thing and while Grit and Panache stack, "grit class feature" is still different from "panache class feature" therefore everything works normally.
d) Amateur Gunslinger turns into "Extra Grit" even if I got levels that granted Panache earlier and not later. This gives me one more grit point but no gunslinger recharge method and no gunslinger deed

...

?

More difficult (and more fun?)
What if my first level was actually a Gunslinger level and then I tried to get the Grit Rogue Talent?

[Yanisa]

Panache and Grit stack and iirc they count as the same thing for meeting requirements.

There's your misunderstanding. They don't.

Let me give you the text, found on the gunslinger (http://www.d20pfsrd.com/classes/base-classes/gunslinger) page.

Grit, luck, and panache represent three different means by which heroes can gain access to the same heroic pool, using it to accomplish fantastic feats. For characters with a mix of grit, luck, and panache, they pool the resources together into a combined pool. (Those who use panache and luck gain twice their Charisma bonus in their pool.) For feats, magic items, and other effects, a panache user can spend and gain luck points in place of grit or panache points, and vice versa.

A luck user does not count as a grit or panache user to satisfy feat prerequisite

That last line is the most important. You can be a swashbuckler and pick up amateur gunslinger, a swashbuckler is not a grit user for the prerequisites of feats. Your closest answer is C, but I go with: "E - Both Swashbuckler Panache and the Rogue Talent Grit function as normal, because neither blocks off the other, except they combine into one pool and you can cross use points between classes"

As for the gunslinger, in that case the Amateur Gunslinger would turn into the feat Extra Grit, because you avoid the prerequisite on Amateur Gunslinger, but not the Special effect. Although this one edges more to DM ruling I think.