Credit where credit is due - I decided to upgrade from the ADSL+ package to self-install Fibre Pro for more speed. IDNet sent me a preconfigured Billiion 8800NL VDSL modem/router, which I tested by adding my ADSL profile to it and using it in place of my old modem - it all worked fine (slightly faster than the old Netgear modem it replaced).
After about a month, the fibre switchover occurred, and I got an email from IDNet confirming this. I'd been oblivious, online browsing, while the switchover occurred and didn't notice a thing...
When I ran a speedtest, my download speed had gone from 10-11 mbps to 74 mbps - with no hassle whatsoever. I guess the modem automatically switched to the VDSL profile.