Hi Matt,
We went through the same process a couple of years ago.
We used the "SharePoint Migration Tool" to move files direct from the network file server to staff/student OneDrives - i.e. we did not rely on folder redirection switch-off to move files to a (single-point-of-failure) local PC and then KFM sync from there to
OneDrive.
The overall order of events was:
Careful testing GPO changes for undo of folder redirection and OneDrive sync w/ KFM.
From memory, set KFM to silently enable ahead of folder redirection remove i.e. it was ready but failing due to the existing folder redirections.
Keeping users in the loop with timing of changes e.g. everyone log off school desktops on particular dates.
Preamble move of files within users' home folders on the server - we still had e.g. Pictures as a subfolder of Documents (Windows 2000 style??) which we flattened out into separate top-level folders, and moved 'everything else' in the user's home folder into
another top-level folder to get all users into a consistent state.
Actual run of the SPMT against a runlist of local server folders to user OneDrives (this was pretty quick; one Friday eve I think).
Turn off file shares for home folders at the same time.
Run script to wipe local profiles on all shared PCs (lab desktops, shared loan laptops etc) to get a fresh first-time login w/ KFM experience on them.
From memory, staff laptops didn't need the abovementioned local-profile wipe for a clean/error-free experience, due to offline files - which were in effect for all redirected folders, but only on laptops.
i.e. each staff laptop user moved at their own pace - from redirected folders (relying on offline files, as the shares were already turned off) to KFM, as they rebooted/GPOs applied.
On staff laptops, OneDrive would have attempted a first-time sync of local files with the (identical) cloud files that we'd put there using SPMT - but we didn't hit any issues with this.
I assume you're using local profiles already; if you're still using roaming profiles then you'll want to make that switch first.