Tear out the Sheppard subway!

I've been pondering the implications of the so-called Transit City LRT plan for Toronto (which has been discussed in passionate and speculative detail over at Spacing).

An LRT is to run westward along Sheppard to Don Mills station, which is great; however, travelling this direction past Yonge will then mean a transfer from the LRT to the subway at Don Mills, then to a bus at Yonge.

It is well-known that the Sheppard stubway, while convenient for a limited range of commutes, loses a great deal of money every year. If the LRT is added, it may being seem like an inconvenience.

My outlandish proposal: rip out the subway tracks and run the LRT underground from Don Mills to Yonge. Eventually, the LRT could hit the surface around Wellbeck (doesn't the Sheppard tunnel already run a certain distance past Yonge?), and head all the way out to Weston with connects at Bathurst, Downsview station, Keele, and the proposed Jane LRT.

Swapping subway for LRT along Sheppard shold cost considerably less than digging a new LRT tunnel (as is planned on Eglinton), requiring only new tracks and some station modifications to raise low-floor LRT doors up to meet the platform. The subway cars used on the Sheppard line are interchangable with the rest, and so they won't go to waste.

Will it cost less to operate an underground portion of an LRT line than the current Sheppard subway? Have heavy-to-light underground rail retrofits been sucessfully accomplished elsewhere? I have no idea, but a great deal of transfer hassle would be saved at Don Mills, and a single-trip, transfer-free LRT run from the east through to Weston would make a lot of sense.