Seems like there should be an upgrade price from V8 licenses that worked for V9. I would have upgraded to V9, but you "gave" some of us V8 users a free upgrade. Now, if you had made me upgrade I could get in on the half-price deal, but since I didn't I'm stuck paying full price for V10? So is this payback for the free V9 upgrade?
You'll never make everyone happy, but if you stick with a more common method of charging for upgrades at least you can claim you're the same as eveyone else.
Something like this:
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Previous Version bought within the last few months before a new version comes out: Free Upgrade
(Sucks in the wafflers who may decide to wait for the next version before buying since it should be out any day now, but who may forget about your product by the time you actually release the new version)
Prevous Version bought beyond the cutoff: Upgrade Price
Older Version but at least X.X: Upgrade Price
Older Version older than X.X: Full Price
No previous version: Full Price
Short, simple, elegant, like everyone else...
As always, IMHO.
-John