I knew there was a neat recognition of the fact that Ray was played by a new actor:

"At the beginning of the second season, however, Fraser took a vacation in Canada, and came back to find that someone was impersonating his partner. In fact, Stanley Raymond Kowalski (played by Callum Keith Rennie) was taking Ray Vecchio's place while Ray was under cover, as was learned later that episode, from Lieutenant Welsh (portrayed by Beau Starr)."

from IMDB
http://www.imdb.com/title/tt0194607/plotsummary