I have had success with doing this. However, the only way it works (I have found) is if you have your plan going into the month lower than what you anticipate using.
For example, our lines use a total of 2-3 GB a month. We always start every month on the 2GB plan, and then on the last or second-to-last day of the month, I check to see if we need to bump up to the 4GB plan in order to avoid overage charges. If we do, it's a matter of simply calling Verizon and asking them to change the plan and backdate that change to the start of the Billing Cycle.
You don't pay any more this way than you would if you had been at 4GB from the start, and Verizon has always been very accommodating in me doing it.
If you have any questions, feel free to ask.