If I go from Reserves (just enlisted) to AD, will I be able to change my MOS before I go to basic, or am I stuck with what I got?

Extremely doubtful- Army does not care what is cool. If you sign to just go AD, then they normally will take you in your MOS, instead of spending more money to retrain you. If you haven't gone to MOS school yet AND there is a bigger shortage of 35F compared to 25B, AND if you are qualified to try 35F- MAYBE.

Depends on the unit you get assigned to AD. If they're long on 25D and short on 35F you might have a slim shot of getting it but if it's the other way around no chance but, again it's entirely their decision so I'd say 50-50 chance if ya pull on the right ears

It’s all about funding and you already sign a contract. They already invested in you with that MOS and it’s the needs of the Army. Talk to your recruiter not sure how involved they are after you already went tk Meps.