Using Encrypted + UnEncrypted passwords

Hello everyone!
Thanks for the forums and for the live community that you are building.

Here is my Idea( I am new here so please.... )

I am planning on adding the forums to an existing website with about 10,000 users.
I have come to the point of decision of removing the encryption of passwords because as I understand there is no other solution in such a case.

I am thinking of using two password fields.
Old_pass, password.
When a registered user user enters the site/forum he will be notified to "refresh" his account - so encryption will be used and his password will be moved from the old password/unencrypted field to the new and encrypted one.

A new registered user will be encrypted.

After a period of 6 months all active users will be moved to the new field and all inactive users will be deleted.

1. What do you think of my idea?
2. I have a hebrew website, are there any restrictions on usernames?

Thanks reading.
DK