Different locations in your house will have different wiring lengths and potentially different parallel loads on the circuit in question that the Old Main fridge is plugged in to. This can throw the detection because the loads are different in different locations.
That said, it is possible, and perhaps most likely, that you lost the detection because the basement circuit is on a different leg (phase) than the kitchen. If you have an alternate outlet in the basement on the same leg you could plug Old Main into that and with luck it will re-detect quickly.
The way to check the phase is to look live in Sense at the Settings > Signals.
There you will see which phase (A or B, “left” or “right” in the display) your devices are pulling juice from as they switch on and off.
If you have a large-ish constant-load device (100W lamp for example) you can plug it into the outlet and switch it on and off and you should be able to identify which phase (A or B) the outlet is fed from. You want to match that phase with the kitchen one where New Main is plugged in.
The other (lazy) option that will probably work is to simply wait until Sense works it out. The previous detection data is not lost and the Sense detection algorithm is using it (along with every other successful fridge detection out there) to refine the detection and account for such things as “Old Main moved to basement”.