The current ships doing Alaska go to Australia for the winter and don't go through the canal. Voyager and Mariner are, or will be, in that hemisphere and could easily do Alaska cruises without needing a canal transit.Though true that RCL's current ships are doing Asia and a trans-pacific, previously they, and most Alaska-bound cruise ships, do a Panama Canal and Pacific Coast trip to get to Alaska. Radiance and Rhapsody can also be moved to the Caribbean through Panama after an Alaska season, whereas the larger RCI ships cannot.
Voyager and Mariner will likely never do Alaska, as not only are they large for the area, they do not meet the environmental requirements (which are only getting stricter). And after arriving, their only option is to go back through Asia/Australia, or around Cape Horn (at least till the Panama Canal upgrade).
I could see Voyager class ships doing Panama Canal and Pacific coast runs after the Panama Canal upgrade.