Th only way that you'll prevent the insulation from "doing its job" is if by covering it, you compress it as well.
How does the height of your insulation compare to the tops of your attic floor joists? If it's below, even, or just slightly above the tops of the joists, then coverin the attic floor with plywood will do no damage whatsoever.
Actually, it will improve the performance of the insulation, as it will reduce covective activity through the batts. Also, eliminating "air washing" over the face of the batts will improve the performance of the batts as well. Eliminating "air washing" over attic insulation is, believe it or not, what Tyvek was originally designed for.
Now, if the batts do stand pround of the tops of the joists by several inches, yes, compressing the batts can reduce their insulative performace.
Advice...if you are going to cover the attic floor with ply, first seal the attic floor. Buy several cans of foam at a home center. Peel back your exising insulation and foam every little perforation in the attic floor. Wire holes, plumbing holes, etc. Do NOT seal gaps between combustive materials (chimneys, stovepipes, etc) and the framing.
Lastly, consider scewing the ply down instead of nailing. If, for whatever reason, you needed future access to the attic floor bays, it's much easier to back screws out instead of having to pull nails.
Lastly lastly...can your attic floor withstand the weight of what you want to store up there? That's usually more a concern if your house is framed with trusses.