I understand your hesitance with going to Gen4 because of the increased data you can get with your current plan. I had that in mind when I wasn't upgrading, as well. Then I thought...do I stay up every night and use my computer for five hours? The answer was no. I could get more data with my current plan, but I never, EVER came close to using it.
Twice during upgrading my computers to 8.1 it didn't work because it didn't finish in the five hour download zone and I didn't have enough free tokens to cover it afterward.
Applying to the legacy plans, the free download period is the "Download Zone", where data used between the hours of 2AM and 7AM does not count against one's daily allowance.
The Gen4 plans do not have a Download Zone.