Mines are not at risk but could be delayed ( Hyperbolae are used to get your attention ).

Conversion from coal to gas means that the energy gotten from coal will be from gas but it still has to be the SAME amount of energy, otherwise less electricity would be produced. So there would not be less electricity generated. Yes eventually it is more environmentally prudent to convert but do it at a time that is not producing hardship for users ( 2014 time line as to go out the window).

Completely shutting down the plant means a shortage in a time when cheaper electricity is needed. I suspect that electricity produced from an existing coal plant would be cheaper than from one converted to gas Because you have to add the conversion cost as over head.

If closing and the electricity produced elsewhere then I doubt that there is 400 million $s saved. I suspect the 400 is from closing the plant but not factoring in the replacement cost.
Replacement means not only added energy used elsewhere but also the added cost of upgrades to transmission lines and delays resulting in that upgrade.

If they intend to close then the transmission line upgrades have to be in place Before it is shut down.

That power is needed now and about as much additional is needed soon. Those are practical and compelling reasons to delay conversion or closure. Politicians and practicallity do not always go together. Practicallity has to trump all else otherwise McGuinty would not have resigned. His resignation is a consequence of placing ideology before practicallity.

My two cents worth says, do not go running scared into the woods from above mentioned political plans and posturing. Cheers, Ed.