Considering the Cardassians wrecked DS9 when they left and the strategic importance of DS9 following the discovery of the wormhole to the Gamma Quadrant and the threat of the Dominion, why didn't the Federation replace everything with Starfleet tech? The only thing they did seem to do was install Starfleet weapons systems. Even after a number of years of Starfleet presence they had to get replacement parts from the derelict Empok Nor station.