Well the idea should be that you complete your vision in 7 days. However minor bugfix and balance updates after the competition are sometimes mandatory - it's can simply be impossible to find every bug in a complex game, and getting balance perfect requires wider playtesting. Of course if you choose an appropriate scope then this isn't so much of a problem!
As for major content upgrades, usually this happens because the developer hasn't really finished their game. Alas, out of hubris they declared it a success anyway. We get a lot of @ on a maps each year, or things that look cool but have no content.
Developers turning a 7DRL into a bigger project is rare, and usually a sign that the developer isn't very good at keeping their scope down (Kornel and Slash being prime examples!) It goes against the spirit of the challenge somewhat, which is to complete something finished and polished instead of getting stuck in the endless update cycle, but ah well, good things can come of it, and the dev can do what they want
For my own game Mosaic I've made a bunch of tweaks that most players probably won't notice. Some were issues I found after release, some were requests from certain dedicated players. If someone gives you their time on the game and asks for a change I feel obliged to consider it! I do have some ideas now for further expansion, but that might not happen. I find it hard to work properly outside of the 7DRL week...