You can change at any time, but to avoid proration, it's best to change close to your plan renewal date.

You don't get charged extra either way, so if you see a special offer that you like, you're best off to change before the offer expires.

One thing I would add to what Sophia said is that you're allowed one change per billing cycle. So if you are anticipating special deals to be available later that same billing cycle (eg. Black Friday or Boxing Day) it may be smart to wait for those. That can be a bit of a gamble though, so take it for what it's worth.