rjm,
I am not recommending the following procedure, but I think you may be able to keep your old thumbnails.
Here's what I did: First I restored an MC12 library backup file in MC13 and checked that it opened correctly. I disabled the Auto-import feature and file tagging and closed MC13.
Then I deleted the MC13 libary files & the corresponding thumbnail files and copied the similar MC12 files (from the same library that I restored) to the MC13 locations. Naturally both MC versions were closed during this.
After opening MC13 everything seemed to be fine and all thumbnails were instantly available. I have runned this test library a few hours and I have not encountered any problems with the features that I have used so far, but I am not promising that there would not be any problems. I haven't tested many of the advanced features like file stacks.
Personally, I wouldn't first try this with a library of 190,000 files. I am now testing it with a moderately sized local library. It has about 5,000 files, mostly audio, but also some videos, images and docs.