Hi Alan,
No, you can't do that. I mean, the changes will be lost when the sync starts. Here are some internals of how it works: Every conversion consists of data cloning and reconfiguration. Reconfiguration is needed to enable the VM to boot. Synchronization keeps track of modified data on the source and transfers only them. However, reconfig has made changes to the disk, too and this can create inconsistency. To work around this, a snaphot of the VM is made right before reconfig and the VM is reverted to that snapshot right before the sync starts.
Regards,
Plamen