It's to soon to say what Apple did internally until someone does a teardown.
From what I understand Apple only did a slight speed bump on the CPU's
As to upgrading, if you have last years (Late '13) I don't see any benefit with what has been disclosed on the Mid '14 models. Opening the system up to swap things would be more costly and it will be awhile before the parts are even available (besides the risk of damaging your system).
If you really want the latest system, sell yours and buy a new one boxed!