I don't believe this is entirely true. Before doing the region change, if you do the system transfer trick (transfer to another 3ds from the same original region) then do the region change (I do mine immediately after the transfer, don't even do the new startup steps on it) then you should be able to access the eShop in the new region and also do system transfers from the new region.Nope.
I've done this successfully multiple times...
First time I had a junker US system I wanted to use to dump Japan transfers into. So I transferred the US profile to another US system, then region changed it to Japan and now I can transfer Japan systems to it before region changing. I also have a Japanese New 3DS that I did the system transfer on, region changed to US, and then I was able to transfer the account from my US system I bought brand new years ago and all my old purchases from the US eshop still work