Modifying a handgun into another legal handgun configuration is legal.
There are no laws restricting an individual or gunsmith from changing an owner’s handgun in one legal configuration to any other legal configuration. (This is regardless of its Roster status or exemption; the Roster is really just a gating function to resale.)
Changes to caliber and/or barrel length can be made, along with rimfire vs. centerfire changeover, conversion between single vs. double-action status, conversion between single-shot vs. repeater status, etc. [However, conversion to any of various prohibited firearm categories such as assault weapon, short-barreled rifle, short-barreled shotgun, unconventional pistol, etc. must be avoided! Both Federal and CA law must be observed.]
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