I think that any heavy rail conversion needs to be looked at as an investment. Sure, you can argue that we spend x million and not see a single new kilometer of new transit, but it can also mean that future extensions become much more affordable. Theoretically speaking, if we converted our entire heavy rail network into monorail, extending or even creating new rapid transit (aka: 100% grade separated) could become much easier and less controversial. Even if we converted it to light rail, we could expand our network at-grade and create a "semi" rapid transit network like they have in Calgary (NOT like Transit City!).
If we were to go with monorail, the good news is that since our subway is high floored, the stations themselves would need minimal renovations. This picture of Tokyo's monorail (I believe) illustrates what I'm talking about:
The bad news is that the straddle design is not fully equipped for our winters. Moscow is the only place I am aware of which has a straddle transit monorail, and it could pose a safety concern:
The good news is that there is a suspended monorail where the running surfaces are covered, so that this problem (not to mention other problems caused by standard rail in winter weather) are avoided. However, retrofitting our subway would become far more difficult. Here are some GSV images of one from Chiba, Japan:
http://goo.gl/maps/ZsGi
http://goo.gl/maps/dwOd
http://goo.gl/maps/STVe
http://goo.gl/maps/gzB2