I have been looking at these upgrades and realized that any purchase would be wasted if I choose a higher upgrade in the future. Present upgrades are based on the 16 units/hour that is the default. I would recommend that if you presently have an upgrade that part or all the cost of that upgrade is refunded if you get a higher upgrade.
This means that if I purchased a Account Fuel Regen Upgrade - (24) at 800 TSC and then later I purchase a Account Fuel Regen Upgrade - (48) at 2000 TSC, my original. 800 TSC is refunded following the Regen Upgrade - (48) purchase.