A painful experience that I hope I can help others avoid.
On 7.7.5 if you export a UDT xml and re-import it without making any changes to it, you will lose all overrides attached to tags using that UDT!!
I learned this the hard way when I needed to make changes to my UDTs to better handle alarming. At first I was changing the UDTs online in designer but when I got to my telemetry device UDT which contains many alarms, I decided to export the xml and make the modifications offline. When I finished re-importing, I found that all my overrides were lost on hundreds of tags. Using an old csv export of the tags from a previous version (7.5.12) and after 10 hours I think I have tracked down all the overrides and corrected them. The users will let me know what I missed over the next few weeks.
It was after correcting this problem that I took a UDT that I only have a few tag instances of (3), exported the UDT and then re-imported it without making any changes that I confirmed that was sufficient to break overrides.
Lesson learned, export your tags to create a backup before editing your UDT offline then after it destroys all your overrides on import, you can import the tags to rebuild the overrides. Hopefully this problem/necessity will be corrected in an upcoming version.