I had a need to populate an attribute with slapmodify and was surprised to find that the entryUUID is also changed when olcLastMod=TRUE. This caused issues with a system that expected those values to remain static.
The slapmodify man page doesn't mention this, and I had to look at source code for an explanation. At the very least it would be nice for the man page to warn about this, but it would be better to have an option to disable this behavior without having to change slapd configuration. I find the defaults to be less than desirable, and this could also cause issues with syncrepl for full refreshes. It makes sense to update the other operational attributes like creatorsName and createTimestamp, but why would anyone ever want the entryUUID to change?