Excuse my stupidity on this.
Like so may other things (BOM for example) it can be done from PCBNew.
The old version imported schematic and .cmp files are associated first using the BACK button.
Next make a new netlist, then run CvPCB to allocate new components from the .pretty libraries, this will then assign the footprints to the netlist. If component footprint fields are still visible these will be seen to have changed.
Next run Netlist again. That updates the netlist to include any changed data.
Open PCBnew and read the new netist.
If all of the components are correctly assigned that is now fine. Spread the components out and check that they are all correct, especially pin assignments on things like TO-92 or TO-220 bases, then save.
If say you need for example a relay that is not in the library use footprint editor from PCBnew to assign the correct component shape and save the file again.
On the File / Export tab there is a tab "Component (.cmp) file". Use this to make the new .cmp file.
(If you want a simple csv BOM as well, go to the Fabrication output tab and select "BOM". This provides a grouped list of same value & type components, with headings for part numbers etc. It is readily imported to excel or open office as a ";" separated file.)
Return to the eschema page, and use the BACK button. Since there is now a .cmp file it will offer the option to keep, hide or show footprint data.
A long way round but it works and is a lot quicker than editing seventy individual parts.
There should now be a fully updated and correct set of files, schematic, BOM, PCB just in need of routing.