Hi Greg,
One way is to overwrite the $PRJ/database/icdb.dat with the new schematic.
Another is to overwrite the $PRJ/PCB/Work and $PRJ/PCB/Layout directories with the PCB you'd like to use.
Having said that, it's always a good idea to make sure all projects you are working with 1) have the same Board and Block names and 2) are synchronized. While doing this merging/copying, it's easier to keep the project and pcb filenames the same and rename them once the operation is complete.
Hope this is helpful.
Mike