Just had a client going from Sage Payroll 2011 on a standalone PC to a network version on the server.
It was surprisingly straightforward! I just copied the Company.001 directory from the PC's payroll directory in to a shared folder on the server.
Then ran the latest Payroll setup CD on another PC and told it to open the payroll.mdb from that shared location when prompted.
The answer to telling the original standalone PC to switch to the network data was prompted by this post http://www.great-one.co.uk/archives/132
But on this version of Payroll I had to copy the paydata file from the new PC to the old.
Advantage of doing this was I could easily revert to the standalone Payroll if there was a problem with the network version.