It is an online conversion - If it is a WIndows machine Converter can perform a sync of the datae - page 71 of http://www.vmware.com/pdf/convsa_51_guide.pdf
Synchronize the Destination Machine with Changes Made to the Source Machine
When you convert a powered on Windows machine, Converter Standalone copies data from the source
machine to the destination machine while the source machine is still running and generating changes. This
process is the first transfer of data. You can transfer data for the second time by copying only the changes made
during the first transfer of data. This process is called synchronization.
Synchronization is available only for Windows XP or later source operating systems.