I'll lead in with this: welcome to Baseops my sweet summer child...oh wait...
WELCOME TO THUNDERDOME
In general: in AMC, you remain in the airframe you were assigned initially. C-130 slick bubbas stay in C-130 slicks. (common misconception: C-130 slick dudes do not easily slide into the AC or MC-130 unless selected). C-17 guys stay in C-17s. 25% of that C-17 force is airdrop qual'd and you'll have to ask an airdrop guy what that's all about...not my world.
In honesty, there a lot of sincerely and deeply experienced dudes on this board. The question you've asked has been addressed many...many...many...many.......maaaaany times, HOWEVER it may or may not have been addressed recently. The environment is changing rapidly with each UPT graduation.
The short answer to your core question: "if I track C-X am I stuck in C-X" the answer is unknown. The real answer is that that the cross-flow program is highly active and highly political...and HIGHLY TIME SENSATIVE. If you fly transport (C-5/C-130), then the tradition crossflow wants to put you into a tanker (KC-135/KC-10/KC46)...and vs versa. That transfer is 100% dependent on luck and timing.
I can't say anything at all about fighter to fighter transfers. I ain't gay, but those tubby bitches love the cock.
Be the best you can at your job. When you're given a job (reads.airframe.you.didn't.want) continue to be the best at your job and you will have options. Succeed and you may find that you're airframe is actually what you wanted (often happens), or that it steppingstone you to where you needed to be.