Will I lose TIG when I switch from NG to AD?

No, your TIG will be your effective date of rank if you don't have a break in service from the RC. Your TIS will be reset to your new adjusted BASD. So, while you may need only a few months more TIG to promote to SPC, you will still need the two years TIS to promote.

You can reference all that in the Promotion Regulation AR 600-8-19. In the glossary section it lists the definition of Time In Service for Active Duty as BASD. There's another section that has the rules for calculating TIG

Pretty much. Your DOR gets recalculated when you go Active Duty, which in turns gives you a different TIG. Being a PFC going active duty, the loss won't affect you too terribly.

I was a PFC when I went active from the reserves. I went active as a PV2 because that's what I was when I went through AIT.