In most cases you would not want to have you metadata messed up, just because KiCad finds another version of your symbol.
However, what one could dream about is a tool that would let you search out for instance all capacitors with a value of 100nF and let you update all the different metadata fields for those symbols. (A nice feature could be that it would only update those fields that were edited,leaving the rest alone.) This would make it easier to enter metadata that can be used for BOM creation.
Currently the most efficient strategy to accomplish the same end result is to create “atomic” parts for the most used symbols, so that the information is already there when inserted into the schematic.