Thanks Rene, I had not fully discovered the table editor, I thought it was just for displaying an overview, but discovered now one can update the fields. However I found the edit to be a bit shaky. Sometime it would take a double click, other time not. And if I edit and apply changes to one field the next symbol I click on will also get that value, so may be some more work is needed there (using unsigned Dec. 8 Windows nightly).
I do think see what Nicholas is after though, One has created a schematic, without much metadate or with incorrect ones in it and now you want to use a pre-existing "molecular" part to get that metadata for BOM/ordering info updated instead of retyping it in again.
For my molecular parts, I have symbol names like R_1206_10K_RC1206FR-0710K and with all needed footprint and ordering info defined. On insertion I change the value to 10K. The part will still reference R_1206_10K_RC1206FR-0710K in my custom library, it is just the displayed value that is changed. If I look at its properties all the original info for that part is displayed and ready for use in Pcbnew or creating a BOM.