Thanks! From what I understand, he's already changed his varnish. From what I understand, he had changed varnishes just prior to doing this painting, and was doing very dark pieces, so didn't realize the effect it would have on lighter colors. It seems to only affect (or is only noticeable on) white areas. Someone contacted Jeremy about this (mine is not the only painting that exhibits this yellowing), and he actually said that putting it in a sunny, well-lit area would actually lighten the painting a bit...but does this make sense to anyone else? I would think it might have the opposite effect. Either way, it doesn't really bother me much, especially since it looked this way when I saw it for the first time...I just hope it doesn't get worse. And I wouldn't mind reversing the process if possible...but I don't even know if that's an option.
I wish I knew enough to give you exact advice on how to fix it, but it's good to hear Jeremy's already switched that.
I think I would be wary about putting it in a sunny area because, while the varnish might lighten, the paint would follow suit- which you obviously wouldn't want because the paint still looks to be fine. Maybe just doing it enough to lighten the varnish, than avoiding the sunlight might work...but that's treading dangerous waters so I don't want to specifically recommend that.
I'd follow the advice of the other posters and contact somebody who deals in restoration/preservation. Even if reversing it (if possible) isn't an option, they could give you some help on trying to keep it as stable as possible. You might want to contact Jeremy to find out exactly which varnish he used to make your job easier.
On the plus side though, that painting might not have come up for sale if it weren't for the varnish issues, and you likely got a great deal on it because of it.