Quite these products are tied to email address used to order so if you use a different email address you are usually ok.
If the web site is sellng a new version rather than just extending updates of virus definitions then you "should" be able to install it over the vesion you have.
Banks normally offer the full version of AV packages. Barclays offer Kapersky free for a year. HSBC, SKY etc offer McAfee for a year. With HSBC if you use a diferent email address you can get a new version every year - that is what I did for multiple PCs for many years.
I currently use the version of McAfee from BT broadband - their offer is free for a number of PCs for the duration of the BB contract.
BT Infinity 2 - IP profile 77 / 20 - super fast!
Previously BE Unlimited - 21,000 Download 1,200 Upload but then moved house - 6,500 Down, 1Mb/s up - gutted!
Ex <n>ildram , been to SKY MAX - 15,225 Download