//sel poly now keeps the previous selection, converting its outline into a polygon if necessary.

This loses some blocks, if anyone cares

src/main/java/com/sk89q/worldedit/commands/SelectionCommands.java

@@ -616,7 +616,7 @@ public class SelectionCommands {
             selector = new ExtendingCuboidRegionSelector(oldSelector);
             player.print("Cuboid: left click for a starting point, right click to extend");
         } else if (typeName.equalsIgnoreCase("poly")) {
-            selector = new Polygonal2DRegionSelector(world);
+            selector = new Polygonal2DRegionSelector(oldSelector);
             player.print("2D polygon selector: Left/right click to add a point.");
         } else {
             player.printError("Only 'cuboid', 'extend' and 'poly' are accepted.");

src/main/java/com/sk89q/worldedit/regions/CuboidRegionSelector.java

@@ -49,7 +49,7 @@ public class CuboidRegionSelector implements RegionSelector, CUIPointBasedRegion
     }
 
     public CuboidRegionSelector(RegionSelector oldSelector) {
-        region = new CuboidRegion(oldSelector.getIncompleteRegion().getWorld(), new Vector(), new Vector());
+        this(oldSelector.getIncompleteRegion().getWorld());
         if (oldSelector instanceof CuboidRegionSelector) {
             final CuboidRegionSelector cuboidRegionSelector = (CuboidRegionSelector) oldSelector;
 

src/main/java/com/sk89q/worldedit/regions/Polygonal2DRegion.java

@@ -50,7 +50,7 @@ public class Polygonal2DRegion implements Region {
      * Construct the region
      */
     public Polygonal2DRegion() {
-        this(null);
+        this((LocalWorld) null);
     }
     
     /**
@@ -84,6 +84,11 @@ public class Polygonal2DRegion implements Region {
         recalculate();
     }
 
+    public Polygonal2DRegion(Polygonal2DRegion region) {
+        this(region.world, region.points, region.minY, region.maxY);
+        hasY = region.hasY;
+    }
+
     /**
      * Get the list of points.
      * 

src/main/java/com/sk89q/worldedit/regions/Polygonal2DRegionSelector.java

@@ -19,6 +19,7 @@
 
 package com.sk89q.worldedit.regions;
 
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import com.sk89q.worldedit.BlockVector;
@@ -46,6 +47,39 @@ public class Polygonal2DRegionSelector implements RegionSelector, CUIPointBasedR
         region = new Polygonal2DRegion(world);
     }
 
+    public Polygonal2DRegionSelector(RegionSelector oldSelector) {
+        this(oldSelector.getIncompleteRegion().getWorld());
+        if (oldSelector instanceof Polygonal2DRegionSelector) {
+            final Polygonal2DRegionSelector polygonal2DRegionSelector = (Polygonal2DRegionSelector) oldSelector;
+
+            pos1 = polygonal2DRegionSelector.pos1;
+            region = new Polygonal2DRegion(polygonal2DRegionSelector.region);
+        } else {
+            final Region oldRegion;
+            try {
+                oldRegion = oldSelector.getRegion();
+            } catch (IncompleteRegionException e) {
+                return;
+            }
+
+            BlockVector min = oldRegion.getMinimumPoint().toBlockVector();
+            BlockVector max = oldRegion.getMaximumPoint().toBlockVector();
+
+            int minY = min.getBlockY();
+            int maxY = max.getBlockY();
+
+            List<BlockVector2D> points = new ArrayList<BlockVector2D>(4);
+
+            points.add(new BlockVector2D(min.getX(), min.getZ()));
+            points.add(new BlockVector2D(min.getX(), max.getZ()));
+            points.add(new BlockVector2D(max.getX(), max.getZ()));
+            points.add(new BlockVector2D(max.getX(), min.getZ()));
+
+            pos1 = min;
+            region = new Polygonal2DRegion(oldRegion.getWorld(), points, minY, maxY);
+        }
+    }
+
     public boolean selectPrimary(Vector pos) {
         if (pos.equals(pos1)) {
             return false;