I use bitcoin-qt and I had an encrypted wallet.dat file stored in a few safe places.
I wanted to check whether I still remembered the password. I copied the stored wallet.dat file into the Bitcoin folder and opened Bitcoin-qt. The quickest way I could think of testing whether I remembered the password was to select the option "create new password", which obviously requires the old password. Upon performing this action and therefore confirming that I do remember the password of the stored wallet.dat I proceeded to delete this "testing" wallet. I have not made any transactions or any other actions with the testing wallet.
However, later on, I have read that every time you change your password previous backups of the wallet become useless. I have run again bitcoin-qt using the old back-up and the funds are still there, however I would like to ask to the experts of the forum whether these wallets are now useless or whether I could face some problems in the future.
Thank you.
If there is not keylogger that could memoryze the password when you wrote it in then i guess it's secure. Otherwise always do a system scan for viruses before you access your wallet.