Thanks Ranochio, I have a knowledge of using command prompt (using cisco command line and windows batch files is no prob either) just wasn't shure what you meant. It's clear now. I'll start bticon-qt with the -upgradewallet and then -rescan as well.
If the blockchain is downloaded I'll try my wallets. I'm indeed not sure if everything was emptied (might have been some 'small' leftovers there, that I didn't bother with at the time, I'll surely let you know how it goes once I've opened these older ones.
I didn't know about the upgade command, so thanks for that.