Since changing your UMS username will also change your User Principal Name (UPN) (basically your email address) for OneDrive, there are a few things that you will need to wait for to happen and a few things you will need to do before you can access and sync your OneDrive with the new UPN.
Instructions
Info: UPN changes can take several hours to propagate and take effect.
- Changing your UPN also changes the URL (aka web address) of your OneDrive, so following a name change you will need to update any short-cuts you have saved to files in your OneDrive (i.e. Desktop short-cuts and browser favorites).
- The OneDrive client (on both Windows and Mac) will automatically switch to sync with the new OneDrive location after a UPN change. If you have issues, you may consider re-installing the client.
- If storing OneNote Notebooks in OneDrive, users will need to close and then re-open any OneNote notebooks.
- You will need to browse to re-open OneDrive files in their new location. Any previous links to the files from before the UPN change (including "Recent" lists in Office apps and Windows) will no longer work.
- Re-share any files or folders you had shared with others from your OneDrive since the old links will no longer work.
Warning: While the UPN change is propagating through your environment, users may see an error in the OneDrive sync app that "One or more libraries could not be synced." If they click for more information, they will see "You don't have permission to sync this library." Users who see this error should restart the sync app. The error will go away when the UPN change has been fully propagated and the sync app is updated to use the user's new OneDrive URL.
Reference documentation:
https://docs.microsoft.com/en-us/onedrive/upn-changes