Well, in your case, with being near the limit.. yes most definately :)
For some though.. if not.. for the sake of billing, etc.. its much cleaner to do it on your billing cycle.
For the bill in the cycle, you will see a full charge for the OLD plan, then a subtraction for the time from today till the end of the billing cycle.
For the new plan, you'll see a partial pro-rated price for only a partial month.
Just looks kinda messy and can be confusing for some.