Re-using old recessed lighting junction box

[SAS]

I am replacing some old Lightolier recessed lights with new, airtight cans. I would like to re-use the junction box from the old lights, since it's simpler and cheaper than pulling the wires out of it and putting in a new box. I don't see any reason that it would be a code violation, but I have a few decisions to make.

First of all, the old box has a BX feed to the old bulb holder (it's the kind that clips into the top of the housing). The BX is not connected with a connector, but is actually "built in" to the box. Do I just disconnect the wires in the box and leave it there? If I cut it off at the box, do I need to seal the hole, and if so, with what? Can I re-use it by cutting it at the junction with the bulb holder and connecting it to the new housing's junction box with a BX connector? If so, what is the easiest way to cut the BX without cutting the wires inside?

Also, the existing box has a thermal cutoff device attached to the box and connected to the circuit. I'm assuming that the retrofit housing (it hasn't been delivered yet) will have it's own. Is there any reason to leave the old one in the circuit?

I have to do all of this from below, since there is no access above, so I'm trying to make it as simple as possible.