GREAT question.
AFAIK the sim. training wouldn't be required; the agreement between TC and FAA remains the same. There may be some guidance recently published that says otherwise - I'm not privy to that stuff. If you know of a FSDO that does a lot of such conversions, they'd be the one to ask. Just ask about the TC to FAA ATP; I wouldn't bring up the underlying FAA to TC commercial conversion.
When I did mine (FAA to TC) I held no Canadian tickets. Since you'll already hold both licenses (going TC to FAA), it's more complicated - but I don't think there are any restrictions. I don't see any below.
http://www.faa.gov/documentLibrary/m.../AC_61-135.pdf