According to Apple, the Apple ID password (which should be the same as your iCloud account and password) must:

Contain at least eight characters, a number, an uppercase letter, and a lowercase letter. You can't use spaces, the same character three times in a row, your Apple ID, or a password you've used in the last year.

If you have uppercase and lowercase letters and a number in your old password, and if there are no repeating characters and not any other restrictions mentioned above, that should be fine. Is that the case? However, if you change the password, it cannot be to a password you have already been using within the last year.

The question is, why is it making you change your password? It's not because the password is too weak. Did you initiate the change? Do they think your account is compromised? Did you try logging in too many times or accidentally press "forgot my password"? Was this totally out of the blue? Do you have any more information about that? If you do have to make a new password, you do have to Make it different from the old one. It's not a bad thing to change your password once in a while, though I understand it can be annoying to get used to a new password and to update your login on all your Apple devices and apps. I remember a few months ago I had to change my iCloud/Apple ID password. I don't recall why or if I initiated the change or Apple thought my account was compromised or if it was out of the blue, but I did have to change it, and it was a mild inconvenience updating my logins.