As the others said; depends on how you do it. If your starting with a freehub with a cassette, then no, as HillRider said remove the lock ring, use spacers and cog and your golden. If however you are starting with a freewheel and want to use a single cog, this will take a bit of knowhow to work; it's doable but the complexity will depend on the freewheel. If you want to remove the multi speed freewheel and screw on a BMX freewheel, then yes, you will need to respace the hub on the axle and re-dish the wheel.