I've done countless EPDM upgrades and there's always someone who doesn't check their data in and I haven't had calls and complaints yet regarding the upgrade itself. Yes we advise them to check everything in, backup the database and archive folders just in case we have to rollback. Can you imagine a user with multiple global sites, hundreds of users and coordinating them to check everything in... Won't happen. Someone will be on vacation or travelling and can't access the vault, yet the latest version of the file is on their local vault view sitting at the beach.
Do your best to get things checked in, make a report of everything checked out (see image below) and then go for the upgrade. Or, wait for everyone to check everything in which could be next year depending on the size of your organization
Files created in new SW release can be checked into the previous PDM release; however, soon PDM is upgraded to the current release, the old release SW files can't be checked into the current PDM
so if the checked-out files are saved to current release, they can be checked into upgrading PDM
as an admin, I always ask everyone check-in their files before the upgrading
I try to get everyone to check their files in before doing an upgrade but with hundreds of users its near impossible. We got it down to 200-300 files checked out from 3000 but I had to go to their managers to get it down that much.
The biggest risk is if the client upgrade on their machines fails and has to be manually removed and reinstalled, you then have to contend with the files that are checked out to that user's local view. General you have to remove the local vault view but keep the files and rename the vault folder. Reinstall and setup a new local vault view after fixing whatever is causing the software upgrade to fail. Then copy the files that were checked out from the old vault view folder to the new one in the correct locations. If they have a lot of files checked out in various folders, this could be very time-consuming.
I have had clients fail to upgrade but never one that had files checked out. I warn the user that they will have to fix this if it happens.