I was reading on Alpha Flight and Fantastic Four and was wondering... since it's a feat card, is it required for these Alternative abilities to always be used in the game? Or can you choose the option to use the original team ability instead?

The old TA is rewritten by the ATA, so if you turn off the feat card the character has no TA to use at all.

Just a quick question. Does one "Alpha Flight" card rewrite the TA on ALL members of your team with Avengers TA, or does one need one AF-card pr model?

From the Alpha Flight feat card description:

Prerequisite: Avengers Team Ability.

All friendly characters with the Avengers team ability have the following team ability instead of the Avengers team ability:

But normal view, I wanted to know if like other feat cards like Rip It Up, etc... whether it's optional to have that feat card active at all times. Say one turn you wish to use Alpha Flight for your entire team and the next turn you decide to turn off the feat card and use the Avengers free movement option.

That has already been answered by clixer11. I was responding to Marauder's question.