Although there's a new preview architecture, the old previews should have been carried over. You may be able to force it by quitting LR, copying the name of the 4.5GB previews, renaming it to something else, then renaming the 500GB one to the 13-3 name you just copied.
If there was an issue with the old previews, this may fail to work, but if there wasn't, you may have all your old previews back. They'll still get replaced in time, so if you've got the originals online, building the previews from scratch may still be a better bet, but this is potentially an option if the originals aren't available.
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