How to manage Office 365 ProPlus Channels for IT Pros

EDubs Try the config.xml below.  

but nothing happens on the screen, and the channel doesnt change in the client UI or in the registry

We don't expect any UI which is desirable as we don't want to disturb end user.  You're right the backstage in Office displaying channel name will not reflect MEC as channel assignment until next successful update. The ODT will change the CDNBaseURL value.  Later, within 24 hours the Office Automatic Scheduled task will evaluate the priority order listed in FAQ and stamp the "winner" in UpdateChannel so they both should match http://officecdn.microsoft.com/pr/55336b82-a18d-4dd6-b5f6-9e5095c314a6.

Note: Since you are moving FASTER->SLOWER, without a GPO the client will have to wait until the build on MEC is newer than currently installed Monthly build.

<Configuration>
<Updates Enabled="TRUE" Channel="MonthlyEnterprise"/>
<Display Level="None" AcceptEULA="TRUE" />
</Configuration>