You’re almost right. When you change the password, all files encrypted with the old password will open automatically. This will work as long as you have been online with AxCrypt at least once with the device in question after the change.
If you’re decrypting on a device that has not been online with AxCrypt at least once, yes, then you can open the file with the original password that was in effect at the time of encryption.
Now to your questions:
1) The same rule applies for permanently decrypting a file. The passwords will work as above. So, if you’ve changed the password and have been online, the new one will work fine.
2) If you do change your password on a monthly basis (which would *not* recommend as a security practice!), you’d wind up with files from a given month opening both the the current password whatever that is ,*and* the password of the month when the file was encrypted. There’s no password nightmare here, since you can open them with the single current password at all times – as long as the device you’re doing it on has been signed in online since the change.
I really think you should consider your password changing strategy, I don’t see it as adding any security. On the contrary, it seems that it would cause a real problem in trying to remember that new password every month, or even worse – you’re using some kind of system to modify your password from month to month. Get a good, long and strong one, and stick with it until you have any kind of reason to suspect that it might be subject to a leak. If this does happen, then you should certainly change your password, and re-encrypt all files that you can. If it does happen, the risk is great though that the attacker has already had access to your files, and copied whatever might be of interest…