I also have a problem related to yours. I use a Garmin gpsmap176 with my motorbike (assisted with a nuvi200) and I have the same problem with the discontinued non-NT maps. If I try to transfer a NT map to the gpsmap176 it will not work.
I also know that it is not possible (or at least known how to) transfer NT maps to non-NT format, and the "market" for this is of course small and disppearing.
On the other hand, I like my old 176 which works perfect especially in sunny weather with its grey-scale screen.
It would be interesting to know if it is possible to transfer a NT-map to a memory card (a special Garmin type card) by the help of a gpsmap276C (or 278) that accepts NT-maps and then put this card into my 176. Possibly the 176 would be very slow due to low cpu capacity, but could it work?