There is a client software upgrade option on the 3015 but I believe it is only for the hardware clients such as the 3002s. I don't know of a way to automatically upgrade software clients.
If you install 2 T1s into the same router, and they are of equal cost, the routing protocol will balance the traffic over the two. Obviously this isn't the same as muxing but it helps. We normally use the DL3800 IMUX by Digital Link. They are pret...