There are a few tools to convert your Exchange BES to to notes. Email Migration: email migration and conversion tools for all popular email systems
You have to wipe out your handheld because the SRP of the server you are currently on is there. There is no way to simply edit this data. Next is that you cant just do an activation: was an account created for you? Wireless Enterprise Activiation / Provision?
I am assuming (which I hate to do so) that your mail boxes are converted over?!
Wipe out the handheld, ensure the BES account for you is created, and then provision the handheld. This is the way I know how to do so.
If you are moving to notes, you need a Lotus Person Document, mail file, BES Account ( a state db is created for you) and then you can activate your handheld.