I used to have a Belgian FN FAL rifle. Unfortunately I sold it 20 years ago and I think the import ban is still in effect. Belgian FNs are going for $3,000 or more. 
I saw a Springfield Armory version that has a ‘thumbhole’ stock. Since the AWB has expired, is it legal to replace the thumbhole stock with a standard stock and pistol grip?
I will post this question on the THR forum, someone there will know.
You will need to replace more than the stock to make it legal. Your firearm will need a certain number of American, non-imported, replacement parts before you can add a conventional stock or otherwise make it into a rifle that would not have been importable.
The importation ban preceeded the AWB and is still in effect.
Losing faith in the Dope, are you?
Yes, but…
The first example that comes to mind is my Glock, the Glock 27. Glock added adjustable sights from the factory to make the import point limit by a very small margin, whereupon they replaced the adjustable sights with fixed sights before they offered the weapons for sale.
Once in the United States, it is my understanding that you can make such a modification provided that you do not violate the provisions of the National Firearms Act of 1934 or the Firearm Owners Protection Act (Hughes Amendment provision) of 1986 by adapting it into an automatic weapon or shortening it to less than 18".
I believe the restrictions and point system on pistols you’re referring to is in the 1968 ban. The relevant ban requiring thumb hole stocks (and also prohibiting the post-import alterations) and such on rifles is in the 1989 ban.
At any rate, I didn’t notice the OP said “Springfield Armory”. I dont know enough about the FALs… are they even imported? Do they assemble them from imported parts? I wonder why it had a thumb hole stock to begin with.
If it was manufactered in the US from US parts, then you can simply change out the stock with no issues.
I have a metric FAL w/ standard stock and pistol grip. So I certainly hope it’s legal. 