Take a system image backup. Cross your fingers. Apply them.
Yeah, I got bitten. Not once, but twice by Windows 10 2004.
My big desktop is running Storage Spaces.
I guess Microsoft forgot to test Storage Spaces.
I saw that my desktop system was offering Windows 10 2004 but I was going through my normal sequence of applying the update to my less critical systems first.
Then I came across Woody Leonhard's article on Storage Spaces. When I went back to my desktop, the update was blocked. And it still is.
You can see Microsoft's support page here.
One of my less critical systems was my former desktop system. It doesn't run Storage Spaces so the update to Windows 10 2004 was offered and I installed it.
When I revisited it a day or two later, I noticed a pop-up complaining about OneDrive.
I didn't think much about it and chose to download files like it prompted me to. OneDrive started copying ALL my files to that system. I didn't think that that was the setting I had so I started un-syncing most of the directories and went on about my business.
A day or two later, I checked back in. None of the files had actually synced and the transfer rate was 0.0.
HotHardware has a good article on it here but there hasn't been much coverage of this. I followed the OneDrive reset (archive.is) process and that seems to have resolved it.
I'm not in much of a hurry to install Windows 10 2004 on my remaining systems.