The only recommendation I have heard along those lines, is the older 2-pin frames should not be converted to .40 or .357 (see post #2 in this thread for an image comparison of a 2-pin and 3-pin frame). The third pin stabilized the locking block better in the frame, and along with a larger/stronger locking block introduced at about the same time, greatly improved durability in the larger calibers over the long term.
The third pin was introduced somewhere in the middle of the Gen3 model run, so you will find some Gen3 9mm full-size Glocks with 2-pin frames, and some with 3-pin frames. I've always been told that 3-pin frames can be safely converted to .40 and/or .357 (given the correct parts, of course).
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