Instead of the '%3A' in the URL you could also just use a ':', that's easier to read and is what I do on Mainly Norfolk.

I've written to the library and have been assured that the permalinks for individial entries like https://www.vwml.org/record/GG/1/7/402 will continue to work (though they are being redirected to, in this case, https://archives.vwml.org/records/GG/1/7/402).

I couldn't yet convince them to still accept requests for the old https://www.vwml.org/roudnumber/ way and redirect them to the new address. I believe the guy at the library didn't understand what I meant - he advised me to use the 'Quick search by Round number' at the bottom of the search page, but of course we want links from our websites to the VWML archive and not enter the Roud number manually in a search form.