I remember when deciding to buy GLBasic, I asked about the cost of updates.
I was told that SO FAR, there have never been a fee for the previous updates, and probably the same for the future updates. That was a nice change from my other software experiences.
But, I was never promised there wouldn't ever be an update fee, and I checked the main feature and purchase page, and nowhere does it mention lifetime free updates. (that I found)
But I can see where people may have bought GLBasic with this assumption and expectation.
And, people may have different financial situations, where an upgrade fee would be undesirable.
But (again) I have to mostly agree with Hark0's ideas.
Say GLBasic has sold 100 copies at $100 (or 90 EUR) for a total of $10,000 (over the 10 or so years it has been around). I have no idea really how many copies were sold, just going off of the number of unique people in the forums. Gernot could expect/get everyone of these people to kick in an extra $20, for only a $2000 campaign.
But, if we all were to promote GLBasic (on social and programming sites for example), and we were to double the sales, or gather 100 new customers, that would earn Gernot another $10,000. So our 'upgrade fee' could be just to try to get others to notice and appreciate the powers of GLBasic. Then at some point it should reach a critical mass, where it can't be stopped, and people everywhere are recommending GLBasic to the new members of the game programming community.
I have left a couple posts on Stack Exchange talking about the merits of GLBasic, when asked for an easy to use game programming SDK. Now, if we all did this, regularly, this could help us all. Imagine the point where Gernot can make enough money from GLBasic where he can quit his day job, and spend his time adding new features and squashing bugs.
As mentioned, we could start with a game programming competition. Game programming has exploded in the past year or two, and I bet you could get a few new customers just doing that.