I trust you mean to say I should use FGU or do you mean to update FGU and copy over to FG classic? (would be suprising if that works)
As for IPv4 vs. IPv6....yes, its rather annoying. I know that about 3 years ago our ISP switched (without asking or info) when we moved houses while upgrading the line - that day FG classic ceased to work (or better for players to connect). Then came the whole Hamachi stuff (which really none of us was happy to use). Now with my current problem here, our ISP "forced" us to replace the router by a newer model and strangely enough I do not even find any IPv6 reference in its setting anymore. Not sure if I can check it somewhere for certain whats used.