Is this actually a one time purchase or are you doing the "Actually if you read the small print I actually mean until I reach version 2.0, then you have to pay again" approach?
At least, the developer should specify how long the product will be supported for security and bug fixes.
A great solution, if you can pull it off: You continue maintaining the base product indefinitely, but "v2" features are disabled until the user pays more. And if you buy v2 in full you get the full feature set.
I have zero issue with developers charging for a version 2, updates, maintenence, etc. Just make it crystal clear.