Ditto both of the above.Two things here, you CAN go back to TE3 with the Bolt. New Bolt - downgrade to HD interface? But, you do wipe out everything, but I'm pretty certain it retains the cableCARD settings.
Also, even though TiVo says that the mini requires 4 tuners, there are many reports of the 2 tuner Edge working with the minis.
The only roadblock to downgrading, NOW (i.e. after upgrading your Mini’s), is if your Mini’s would be blocked from downgrading by the downgrade bug reported in a few threads.
As for the EDGE, unless you’re a fan of TE4, I’d opt for a 4-tuner Roamio before considering an EDGE.