I think I was having a senior moment when I posted in this thread earlier.
I had forgotten that I rebuilt my database and version of phpdig from scratch. Sorry about that. I didn't want to leave the impression that I had just upgraded from 1.8.3 to 1.8.6. I have a lot of customized code in mine so it's just easier for me to rebuild from the ground up to make sure all code changes are applied. Even so, I don't think upgrading would result in any lost data for you.
One of the things you might consider if you don't already have it in place is a mirror of phpdig on a test server, or at least a test folder so when upgrades like this come along, you're able to ensure that any changes you apply won't clobber your production application.
I should also mention that one of the things I did while performing the upgrade was to replace my phpdig search page with one that basically said that search was unavailable pending the upgrade. I think it's a nice way of not leaving visitors in the lurch.