Mathew Newton wrote:

If there are other benefits in your doing so, many ADSL2+
modems/routers can be made to operate in ADSL1

Yes, might wring a bit more speed out of the line than the Cisco 803
does, I suppose.

or alternatively your ISP should be able to force the connection
type.

Yes, I've seen the ISP offering to downgrade people back to ADSL1, with
the proviso that one day they'll be dropping ADSL1 service so have to
upgrade again, I'm hoping for FTTC to beat that day :-)