Sorry, but older serial numbers won't "unlock" the ad-free v3.0.
Please understand, since FeedDemon was created way back in 2003, we've only charged for it one time. The upgrade from v1.0 to v2.0 was free, as were all the smaller upgrades between v1.0 and v2.0, and v2.0 and v3.0. There have been almost 20 releases since v1.0 (some minor, some major), all of which either used the same serial number or were free.
And, of course, we didn't even plan to make a paid ad-free version. Honestly, we didn't expect much demand for it, so we were surprised to hear so many people say they'd rather pay than see the ads - and it's hard to ignore people asking to pay you :) So, we ended up making a paid ad-free version to satisfy that demand.
PS: Your serial number will work on multiple computers, so you don't need to purchase a serial number for each computer you use. There is a to-be-determined limit to how many computers you can use the same serial number on, of course, but we'll be generous with that limit.