In addition to what Andrew has said, it may be more efficient to change the instHeader for the instances that you want to all change to a new master. Find the instHeader, something like:
ih = car(setof( instHead cv~>instHeaders instHead~>libName == "origLibName" && instHead~>cellName == "origCellName")
dbSetInstHeaderMasterName(ih "NEW_PROCESS" ih~>cellName ih~>viewName)
[I have not tested the above code, but it should work]. If the cell names are different, then you might want to have a mapping table for the from-to name mappings. There are a few solutions in SourceLink for doing this too, for example:
Take a look at the last two in particular, since they use the instHeader approach. Many of these will change multiple cells, or even a whole library. Did you look on SourceLink?
If you need to re-reference all the cells to the same new library, why don't you use Edit->Rename Reference Library from the Library Manager menus?