If you manage to clock-up 50,000 miles on star alliance, TG gives you an upgrade voucher independently from your mile count.

They have also introduce a birthday upgrade voucher which you can use during the 3 months period around your birthday (birthday month + one month on each side).

This means that if you qualify in a year you get:

Your normal airmiles count
one upgrade achieving Gold Status on Thai
one upgrade for getting older

I would suggest you book your premium economy and then upgrade to C.
I have had no problem in the past redeeming upgrades on TG even at short notice.

If you want to keep your mileage count, you can also pay for stand-by upgrade at the airport (not sure it is available at JFK). These are reasonable and they sometime do additional promotion.

I just returned from a SIN => ROM => SIN via bangkok in F for 112,000 miles. A bargain.

So no second thoughts, go and join