Upgrading an older License of FCP more than once

[jimbojimjim]

I picked up a used copy of FCS2 recently, however, I am now needing to upgrade it so that another editor and myself can pass projects back and forth without version compatibility issues. I have no way of knowing if the previous owner ever upgraded it. If I buy the upgrade version of FCS3, is there a chance that I won't be able to upgrade my used copy of FCS2? I'm worried about having to spend 300 bucks to buy the upgrade and then have to spend $1000 more to buy a new copy if the upgrade doesn't work.