migrating from mailenable to exchange 2003

you mean that you are doing mailbox enbled user from mail enabled  user in exchange 2003.

The procedure is simple:

In ADUC go to user account Right click go to Exchange Tasks ...Create Mailbox and chose the Mailbox store name where you want to create mailbox then click next .. next and finish.Mailbox will be created with all three tabs Exchange ADvanced, Exchange General and Exchange Features.

Thanks

If yoy are getting Exchange attributes there on mail enable then do steps like this:

Go to user account in ADUC ..Right Click go to exchange Tasks....Remove Exchange Attributes ...next next Finish and then follow above steps.

Then as email stmping may long you right click on recipient update services in exchnage 2003 then click update now to do email stamping on user's mailbox fast.