After 2 years, why give it up now? I wouldn't consider it being rude, and I'd hope they wouldn't either. It's a personal choice. It's not something you dreamed up the day before Thanksgiving so you could inflict drama on the hosts. It's not like you looked at the turkey and said, "Oh, yeah, that looks gross. Pass the mashed potatoes."
Just be polite and decline the meaty items. Explain your position without too much detail. I'm sure there'll be plenty of other foods you can eat there. If they're decent people, they'll respect your decisions and you. If they find it rude or offensive, then they're not worth the trouble. Don't be something you're not just to please other people. You shouldn't feel pressured to compromise your principles because someone asked you over for a meal.
Should you choose to eat it: Everyone is different. The meat may not settle well in your stomach after so long without it. It's not like you'd be violently ill or anything, but you may have a little uneasiness. Or, you may feel completely sick and have to excuse yourself to the restroom. Or, everything may go fine. It may also be more mental than anything. As long as you aren't grossed out by it, it may all be okay.
I've been a vegetarian for a month shy of two years, and I wouldn't consider eating meat to satisfy anyone else. I was invited to a Thanksgiving dinner this year as well. I made sure my non-meat-eating preferences were known up front so it didn't appear rude, and I was informed that it wouldn't be a problem. They understood and respected my view, even though they didn't feel the same way. The only thing they asked was if it would make me sick or if I would make a scene if they ate it. I assured them I respected their opinions as much as they did mine. As long as no one tried to force me, I'm fine with everyone making their own decisions.