Hi sorry for the late reply. I basically took my custom made 3.9 kick and merged it with a vanilla 3.1 image. I then wrote the resultant 2x1MB kick file for lo and hi to the relevant Rom using a willem programmer. I then connected a switch to pin 1 of each Rom. The switch feeds either 0v or 5v to switch the banks from upper 512k or lower 512k. I made sure the kicks were merged with 3.1 on the lower part and 3.9 on the higher part. Then the switch simply selects whichever ks environment I want to use.