Think I got the answer here:
/*
"ripup" all the signal polygons on a board. (that is,
return them to their unfilled state; this shouldn't
actually ripup any routed signals unless you happen
to have a signal at the exact start point of the first
wire of a polygon.
*/
board(B) {
string s;
string cmd = "grid mm; ";
B.signals(S) {
S.polygons(P) {
P.wires(W) {
s="";
sprintf(s, "ripup (%f %f);", u2mm(W.x1), u2mm(W.y1));
cmd+=s;
cmd+="und;";
}
}
}
cmd += "grid last;";
exit(cmd);
}
It rips up all polygons on all layers, and undo's every time. The undo is
undoing eventually wire rip-up. Use on own expense!
It seem to work for me, but no guaranties 
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