Well, I've just changed my username, and just as I was about to change it back, I received an alert saying that I can only do so thirty days afterwards. Could it be made so that the period is shortened?
No. Changing usernames may confuse other users, especially when done multiple times, that's why there is a waiting period. See Can we restrict the ability of users to keep changing their names? on Meta Stack Exchange.
I guess Stack Exchange could reduce the period from 30 to, say, 20 days. I'm not sure if that really solves your problem. Anyway, that should be a network-wide change, it's not something we can decide on Chess Stack Exchange alone.
Personally, I'd rather get rid of the ability to change your username entirely and make it possible in extraordinary circumstances only.