Anyway, I was thinking, would be it an idea to have a 'per version' upgrade fee? I guess it's sort of like what vB is doing but XF could do it differently. You'd have three pricing brackets (per 0.0.x upgrades, 0.x upgrade and x upgrades).
You'd keep the $40 upgrade for a years support too, but you also get this added if you're a bit short of cash but want to keep the support going. Also, depending on how you choose to upgrade, you get support until that version is no longer the latest.
Just my two cents, I will be upgrading for another yearsworth of support whatever happens, but thought it might be an idea.