Does it depend on availability of a MOS or your ACFT (ASVAB retake) GT score to change MOS?

All the above- most MOS's have GT/ASVAB limits but if the MOS is overstrength then I won't be available in the 1st place,

Both. You'll have to look at the in and out calls for your MOS and the MOS that you're trying to re-class into. Unless it's one of the packet MOS's then I believe that the in and out calls don't affect those. You're best bet is to go and talk to your CO/BN retention NCO or career counselor. They'll be able to give you definitive answer.