In the Home Patrol sense, its just overwriting the whole database every time.
In the PSR-800 EZ Scan it too overwrites the library every time and if you "import items into scanlists" and you upload the library again, the PSR will essentailly have updated all items in your scanlists previously imported from the library with new data for the items you imported from the library.
However, for something you have imported into a scan list already if you change something in the name or tag after you have imported it, there is a checkbox in the software that during a reimport of that system grabbing all the new changes in the library it gives you the option where you can choose to keep or overwrite any custom name changes etc.
I too am now wondering if there is something similar in anyones thrid party program for the PSR600 or the BCD996, etc. So, for example, if I have built an extensive list of 10,000 channels and/or 400 systems and a crap load of talkgroups and I want to update any changes - only overwriting any new talkgroups, sites, control channels, names, etc. This would be a cool feature if someone had it.