I presume you are placing a component on your schematics and then editing that component properties, e.g. comment set to 4K7 or 100nF etc.
If you work that way, then update from libraries you lose those comment values you entered, and any other new fields you entered for the component (i.e. when double-clicking component in the schematic).
One workaround for this might be to create a library from your schematic components by drag/drop them into a new SCHLIB although I have not tried that.
IMHO there is a better way to create schematics:
Each component in a SCHLIB library should be unique, it is one orderable part.
One component for every resistor value, every capacitor value etc.
It takes more work up front but becomes a joy later when you have a reasonable library of 'jelly-bean' parts, create some new parts for each job, drop them on your schematic and your parts list is done automatically.
Make any changes in the library version of the component (NOT on the schematic) and then update the schematic part from the library.
For example:
My library of common Yageo 805 and 603 caps / resistors which all have Farnell and Digi-Key order codes:
https://www.eevblog.com/forum/altium/altium-standard-resistors-capacitors-and-inductors/msg840139/#msg840139