What advice do you have for someone reclassifying to 35G? Do I have to have SIPR Token before attending AIT?

Go onto the ATRRS course catalog, find your MOS, and look at the special instructions for your course. Chances are, being an entry level course, you can show up with just your clothing bag and be ok

It's also in your ATRRS instructions.

When I reclassed from 12B to 35F we used a username and password at the school house and no need for a SIPR token until we arrived at our unit. That was nearly 10 years ago, so I'm sure things have changed. Speak with your career counselor.