I wouldn't say it is a refit of the Constitution just yet. I have always had something hoping for a revival of old design philosophies in new ships - I keep expecting someone on BCCentral to make a new Enterprise that heads back towards the saucer-neck-pylon-nacelle profile made so famous by the TOS Enterprise. It maybe that because the Klingons have become hostile, that intrepid pioneer spirit of Starfleet has come back, heralding nostalgic feelings. And somewhere along that line of thought, someone made a workhorse ship that is an homage to the good ole Constitution. I am very happy that they revived the Constitution look for STO.
Back to the idea of refitting a Constitution; how many Constitutions do you think they'd have left to refit? If anything, those parts would have been used for Miranda's, which in general seem to be more advanced than Constitutions.