Exporting & Importing is VERY easy. How many galleries do you have in there?
The Search & Replace thing - I just looked an
in theory this
should work:
1 - Back up the database
2 - Go to Search Galleries & on the top right click the Quick Tasks icon (looks like a wrench/tools)
3 - On the top Search and Replace, select Thumbnail URL. Then search for "http://" and replace it with "https://" (include the :// so that you don't change existing "https" to "httpss"
4 -
to
and do a Build
If it worked, then
If it didn't, Restore From Backup & go from there