If I enlist active duty at the end of my NG contract, would I have to start over rank-wise and go through BCT and AIT again?

NO as long as it is the same MOS.

It is normal to keep your rank, don’t let them BS you into thinking you have to give up a stripe, they tried to do that with me. Through some regs at them and they were happy to let me sign. No additional basic, but maybe AIT if you plan on changing your MOS.

I just switched back to after 2 years in the Guard. You are able to drop a conditional release after your first year is through and do not want to until your contract is up. You may have to reclass or just check to see when your MOS is under strength or balanced on the In and Out calls. I got to keep both MOS and Rank how ever my DOR was changed from the actual promotion date to the day I returned to active service.