Well, I was able to switch my FFN at the gate. The agent didn't give any hassle but did warn me that it was possible that TG would revoke the miles I'd already earned on the outbound portion which was flown a couple months prior. I just checked today and the miles did indeed credit to OZ. Checked TG website and those miles she warned me about are still there (will check again later as I don't know how long it would take for them to claw them back).

So my next question is, which Asiana credit card should I apply for?

Asiana Airlines Visa Signature
30,000 Bonus Miles after $3,000 spend
10,000 Bonus Miles Certificate awarded annually*
$100 credit toward Asiana flights per year (I don't expect to make use of this often)
2 Asiana Airline lounge invitations (do I have to be flying OZ to use them?)
Mileage earning:
3 Miles on Asiana Airlines purchases (probably won't get to use much)
2 Miles on gas and at grocery stores
1 Mile on all other purchases
$99 Annual Fee

Asiana Visa Business Card
10,000 Bonus Asiana Club Miles after first purchase (seems like this just happens once, versus every year on the Signature)
$100 credit toward Asiana flights x2 per year
Mileage earning:
2 miles for every $1 spent on Asiana Airlines ticket purchases (don't expect to make use of this often)
1 mile for every $1 spent on all other purchases.
$80 Annual Fee

To me it seems the Visa Signature personal card is much better if my primary goal is to earn and maintain *G status for travel on other SA airlines. Does "10,000 Bonus Miles Certificate awarded annually" mean that the threshold for reaching and maintaining Diamond tier on OZ is reduced by 10,000 (or even 20,000 since it's 2 year intervals and I would get the certificate annually)? If so that would be fantastic, meaning it will only take 30,000 miles flown on SA to earn Diamond and 20,000 (or even just 10,000 if consecutive bonus certificates apply?!) every two years. This would be well worth the $99 AF even if I'm unable to make use of the offsetting annual credit toward OZ flights.

Am I dreaming that it's this friendly of a FF program? Someone pinch me if I am.