I think that Michael is right. Whatever clock source you select needs to be present for the device to do anything, including ISP. So before removing the device from the board you will need to reprogram UCFG1 (the device configuration) to use an on-chip clock source. You can create a simple little Flash Magic batch file to do this if you wanted. Then all you need to do is to remember to quickly run the batch file before removing the device.

Andy
ESAcademy