I would imagine this could be a C+ ---> change if aircraft was changed to one that only has PS. Sounds like DL updated a few routes with re-furbished 777s or potentially A359s (unfortunately not my SYD - LAX next April).

Wait for the dust to settle and then you can call and see what they'll do -- just search for options that still have C+ (if it holds).

They should refund you the difference or alternatively they may give you the option to a different route. The most logical would be NRT - SEA - PHX with a 763 on NRT - SEA that definitely will still have C+.