Fake your way through Guided Setup, telling it whatever lies are necessary, until you get out of it, so that it thinks you're still on analog cable.
Then tune to the channel to which you have the DTA set (if there's a Channel 3 in your old analog lineup, put the DTA switch on 3, and tune the TiVo to 3, if you have a Channel 4 on your old lineup, set the DTA to that,
and tune the TiVo to 4)
It needs to be able to see the output of the cable box on the tuner (or, if you had a cable box with the red, white, yellow jacks, on the input jacks) when you're doing setup.
Then go to Settings, Channels, Channel list, read the small print at the bottom of the screen and do what it says, hit Enter (little button, bottom right corner of remote)
That should let you go through the cable lineup setup again--tell it you have a box and it's digital.
If you get to the what brand page, tell it Scientific Atlanta.
It'll try about 10 different codes (without letting you know which ones) until it finally gets around to 10014, which is said to be the one that works with that box.
The Channel Changing option, where you can actually tell it which code to use, doesn't appear until you've more or less successfully got channel changing already set up.
I've been through this with the Cisco 170HD DTAs from TWC on my s2s recently, so the memories (and the scars) are still fresh.
"I am altering the deal. Pray I don't alter it any further."
Darth TiVo, 14 February, 2011