Copying from my post that BatBoy just helpfully referenced:
xdslcmd configure --maxDataRate 50000 25000 75000
didn't seem to force a resync by itself.
But xdslcmd start --up --maxDataRate 50000 25000 75000
did the trick.
I can confirm that my line has remained very stable since I made the change on the unlocked HG612 to lower the sync speed. I have had one reconnection; not sure why that was. The router held its setting so I am still with the higher snr (14.8 down right now).
This isn't really proof of much as I had a pretty stable non-interleaved connection even at 70Mbps or so with around 6db SNR. (Helped when I had a new wire from the pole after local tree work killed the old wire. My stats graph has been really boring since!) My non-interleaved ping to bbc.co.uk is typically 11ms; 12ms right now.
Recently moved from BT Infinity 2 to PlusNet. Very happy so far.