That wouldn't work, actually ... when I add in the Redirect as written above, it causes Apache to crash, as a Redirect is supposed to be to another URL :(
Putting the two ProxyPass entries in does help things though, but he has references hardcoded in his HTML to /my1stsubsite, which, of course, the ProxyPass entries "hide", so he ends up tryign to "Proxy"
http://www.myolddomain.nl:81/my1stsubsite/my1stsubsite, which, of course, fails ...