I registered for housing around September, I chose Hullabaloo as my 1st choice and apartments as 2nd and 3rd. Now I am thinking of switching to a less expensive dorm or going to Mosher (Engineering Dorm). How long do I have to change it before it is set in stone? Any recommendations on a good dorm that is decent and affordable?
Not sure about changing your options right now because housing is full.
I would guess you’d have until May 1st, to be safe. Only because that is the cutoff for deciding which college you will be attending in the fall. I would not give up hullabaloo until you are sure - it is the most popular dorm since it is only 2 years old. Mosher is in the Commons - which was built over 30 years ago-just like the modulars. Your previous choices seem to favor the newer properties - since the apts will open this year. Prior to Hullabaloo being opened the Modulars & the Commons were the two most popular,so if you’re looking to go down in price that would be the next logical choice. There are lots of photos online if you can’t visit. Given your date of preference you will most likely get your #1 choice, so choose carefully 
you can call housing and ask about a last-chance date for preferences. Housing being full has no bearing on your preference choice since you’re already in the group that has been promised a room.