If you had passive pickups, then switching to active only means that the output impedance driving the input will be lower than it was before, and this is a good thing. The only exception to this would be if you had a preamp that was designed to interact in some way with the impedance of a coil, in order to achieve a certain tonality. This is rather rare. That aside, the only issue with having an input impedance that is too high, relative to the load that drives it, is that you can end up with more noise. That's not an issue here, however.