I've been thinking about writing some software that will make it easier to run the business, I'm poor and can't afford to buy commercial software for a photo business. However if I spend much tyme programming then I'll want the option to sell the software to other photographers yet prevent them from being able to turn around and sell the software themselves. However as I improve the software I'd like to open the source of older versions.
Why not adapt some FOSS software? Because you want/need the copyright to "protect" you? Is it not better to cooperate with other photographers to develop the software - in that way you might not only get better software, but also get more time to photograph, which seemingly is the point of the whole software thing.
FOSS developers tend to share to reap the benefits of a cooperating group. We collaborate in creating something of a much better quality and feature set than any of us could do on our own. I've contributed quite a lot of code to GPL'ed software, most of it "owned" by commercial vendors. Putting a price on everything is not necessarily the best solution. Don't you ever work together with your neighbours to fix the playground? Do you require ownership of the parts you fix? Do you demand that all neighbours pay a fee, then divide it to the ones working? Where I live, not even all that meet up do work - some are in charge of the barbecue or coffee...
You should also be aware that if you GPL the "old" versions you have, some other people will likely improve on your software and start using it. As your "newest" version is NOT GPL'ed, you cannot apply their patches. So, you will most likely end up with a competing, non-free version. If your program is any good, you will end up with the most expensive and likely worst of the two.