I am using:
You'll need to delete the CTI RP first. Once that's done, you can add the DN with the correct VM profile to the device and move on to (I'm guessing) Unity Connection.
Remove the forwarding routing rules and either change/remove the call handler extension or delete it all together (I normally keep the call handler for the next time someone changes their mind).
Then just create a voicemail box as usual and use the correct extension for the mail box.
These are the instruction I put together for myself originally but this is the first time I will be doing it.
1. Delete the CTI route point in CUCM
2. Go to the phone I want to add it too and add it as a second line, set to both ring and light
3. Go to Cisco unity remove the call handler under call management/system call handlers
4. Remove all call routing in unity under call routing/forwarded routing rules
5. Create a voicemail box for the second line on the phone