I'm not sure how they do it in the Reserves, but having been discharged from the Guard in 1992, I looked a few years ago at reenlisting and asked if I would have to go back through Basic and AIT. I was told that anybody who's already been through Basic or bootcamp is exempt. Not sure if that's really true or not, but it did make sense. I don't know very many vets who would want to go through it again. Personally, I know I couldn't put up with the screaming, physical, and mental BS like I did when I was 17.
As it is, given the fact I have a bad back and arthritic knee, there's no way I could pass the physical just to enlist.