Quote from: julius on March 22, 2015, 07:51:08 PM

-snip-
However, later on, I have read that every time you change your password previous backups of the wallet become useless.

This is only true in certain cases and only for newly generated private keys. You however did not create a new key, thus your old backup is still valid.

Quote from: julius on March 22, 2015, 07:51:08 PM

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 the forum whether these wallets are now useless or whether I could face some problems in the future.

Thank you.

As long as you create a copy of your backup no matter what you do with said copy will invalid your original backup. It will still have all the keys it had before. The only thing you potentially could do is use the keys in the copy and e.g. send change to a newly generated key. This would not make your old backup invalid (still has the same private keys), but it would be without bitcoin (those are only spendable with a new private key that is not part of your original back).

I hope this clears it up, if not ask.