This is in a new MBUX model? Weird, I've done the same process here in Australia and the update didn't mess anything up. (Although I imagine my maps are different to yours, and I never received an email from MB telling me there was an update)
Yes, new model I believe they called it the 2020 model but I picked it up in October 2019.
I didn't get a chance to try it this morning.
This was part of the email
Part of our offer for you is the "Online Map Update" service.
A new version for your navigation maps is available at the Mercedes me Portal.
You can download the maps from the portal to bring your navigation system right up to date.
If you receive automatic map updates for a certain region, this will continue to work in the customary manner.
Your Map Update Team
So I logged in and followed the instructions to download the download manager and copy the files to a memory stick.