GOG will upgrade stone if it is $500 more. There is nothing about value on WF site that I can find.
Does anyone know... does this mean WF considers any upgrade as long as the stone costs even as little as just $100 more than first stone(minus shipping)?
"Do you accept trade-ins if the diamond was purchased from Whiteflash?
Yes. We have a lifetime trade-up policy. We will credit you with 100% of the original diamond purchase price towards a diamond of greater value."
Here is a link to where I found the info. Hope this helps!
Credit 100% of the purchase price (less shipping) toward an item of greater value.
Only AGS/GIA stones are eligible for trade-in, and trade-in stone must be accompanied by original grading report (or pay $200 regrading charge).
Setting charges may apply.