Switching (Converting) from "extentions" to "deviceanduser" mode

I have an established system that I would like to convert to Device and User mode. I have 9 extensions that will need to become users.

If I change the “User & Devices Mode” setting from “extensions” to “deviceanduser”, what will happen to the existing extensions? Will I loose existing voicemail recordings and messages?

Thanks,
Mike
FreePBX 2.9.0.12