You'll need to check out the individual vendor's policy.
My jeweler will take a stone we bought from him and give us the full purchase price amount toward the next purchase. He didn't require that we spend X amount more or anything, but I know some do. This is what we did when I got my upgrade stone.
I was told at GOG that they do not normally do that. However it may be worth asking still as each case is different. Especially on their patent cuts, doesn't make sense not to as they stand behind their stone and should not be concerned with not being able to sell it again, it would be more likely that they would honor an upgrade even if the owner changed.
It depends on the vendor - one vendor I spoke to said that if it's a RB and within their specs (i.e a stone that's easier to sell) a trade is possible, although I don't know how much the offer would be.
The policies are given to the original buyer. I doubt they would automatically give 100% of original purchase price to a second-hand buyer unless it was to their advantage to do so. My guess is that they won't make a blanket policy but deal with each stone individually. So find out for us, DF!