basicly when you get a gun annodised it need stripping1st which dissolves the old anno but also dissolves some of the alluminium, im talking microns then its reoxidised which removes more aluminium (microns again) then its put in dye which adds a few microns but not as much as has been taken off in the 1st 2 processes.
Now that may sound all abit technical but to cut it short, reannoing your gun can cause problems if its not been done right
i.e: if they take too many microns off your orings wont be sealing nicely and will leak, ive seen this before on someones gun they sent to a commercial annodisers