I see. Doesn't say much, not to me at least, as I live in Sweden and more or less have no clue about how the TV networking works in the states.
NBC and NBC Sports are separate networks.
When a game is on NBC, no other NHL game can be played during a 3-hour window. Today it was 12:30 p.m.-3:30 p.m. ET because of the DET-CHI game today.
Because Washington played last night, there is generally a 21-hour window to when a team can play the following day. Tonight's game could be changed to a more convenient time than a 7:30 p.m. start, which is what NBC Sports prefers, when the network went with another game. Considering it is Easter Sunday, it was changed to an earlier time.
Maybe too much info, but an explanation as to the change.