Usually, when you change your password, you have to enter your old password. With both plaintexts, the check is trivial. It also means it should be possible to

Usually, when you change your password, you have to enter your old password. With both plaintexts, the check is trivial. It also means it should be possible to bypass that by changing the password twice or by "forgetting" your old password. Another possibility is that they simply lie to you and the rule that is actually checked is much more permissive. I've often seen requirements that are not actually checked