Most folks will aim for 1-1-11 but the mandate is 1-1-13.
Usually a wideband can hear a narrowband (low audio) but the narrowband cannot hear the wideband, it is best not to mix them, once a system switch, they should switch every thing (system and mutual aid frequency's) then training and radio checks by neighboring agency's should be performed.
Unfortunately there will be mobile/portable radios wideband well past the 1-1-13 deadline, causing interference, (itinerant, radios sold at auction, and some that just cannot afford a narrowband capable radio)
Hopefully this takes care of your some your questions:
narrowband