If you switch from any branch to Marines, do you still keep your MOS or can you choose a new field to be in?

I'd worry about getting through boot camp first. I agree with GySgt Olson - your MOS will be assigned dependent on the needs of the Corps at the time. Based on your Guard MOS in Motor T, you may be able to keep that - the Corps always needs drivers and mechanics. But you may also go into something entirely unexpected, like IT. It depends on how you test when you enter the Corps.

Both MSgt Orvis and Gunny Olson are correct. The needs of the Corps will be taken into account, but when you sign a new contract you will have some say in what MOS you will have. That being said, you will have to go through Boot Camp again, then MCT (unless you go infantry then SOI) then onto your MOS school. We had a prior Army guy in my platoon in boot camp, and while some of the things he did in the Army transferred over, rank (although he did go from an E-3 to an E-2), some of his ribbons/medals most everything else he had to do over again. It is by all accounts a fresh start. DO NOT let the prior service recruiter pull one over on you, make sure you research everything before you change branches.

You get what you negotiate!