I have a Sig 556 Classic SWAT with the quad rail, has anyone every swapped one out for a polymer rounded one? Will the ones sig still has for sale fit or do I have to pull some jedi mind trick to get it to fit ?

Do you have a standard length or Patrol length? The polymer handguard fit into the gas block and that doesn't change so yes, they'll fit as long as you have a standard model and not a Patrol length (short) gas system.

I have a standard length. Thanks. Gonna grab one since I am hanging onto this rifle

it actually doesn't work. the sig swat classic rail extends the entire length of the rifle so changing rails means removing the picatinny rail for the entire rifle

Well, yes. Just put a normal rail on top or an MFI diopter sight/rail combo.