which is why I was wondering if I could change it and if so, if I had to wait a certain amount of time.
Sort of depends on what you mean by 'changing it.'
Some hosting plans like the 'Special' plan let you have multiple domains ... so then it'd be a question just adding that domain and letting support for the other one lapse.
I can't say this with any authority, but with the 'profi' plan you'd prolly have to pay some sort of fee to change primary domain, but it's not in their interest to keep you 'locked' into that domain. It's more money for them and a happier customer if they can just charge you a fee to switch primary domains. At worst you could pay $3.95 a month for an 'addon domain'