Is it possible to retrain at say level 12 from a Warpriest to a Cloistered Cleric or vice versa?
Yes, you can generally retrain almost everything and almost completely rebuild your character from scratch, with a few noted exceptions like ancestry, heritage, background, class or ability scores.
It is dependent on GM approval in that your GM decides whether you can even get retraining, and if so how much it costs and how long it takes.
Yep, takes a bout a month.
Well, technically, even just changing from Warpriest to Cloistered Cleric takes "at least a month" of downtime, possibly more depending on your GM's decision. And that's in addition to the time for all the feats, etc. that you might want to change to fit your new build.
What about accessibile dedication which wouldn't have been available due to lack of requirements?
When retraining, you generally can’t make choices you couldn’t make when you selected the original option. For instance, you can’t exchange a 2nd-level skill feat for a 4th-level one, or for one that requires prerequisites you didn’t meet at the time you took the original feat. If you don’t remember whether you met the prerequisites at the time, ask your GM to make the call. If you cease to meet the prerequisites for an ability due to retraining, you can’t use that ability. You might need to retrain several abilities in sequence in order to get all the abilities you want.