Colt changed it just before WWI when they started making the 1911 Government Model; and for the reason Old Fluff states, commonality of barrel making tooling. In any case very few guns were made by Colt using the smaller bore size - because nobody wanted to buy one. SAA production was rarely above 1000 guns a year during the 20's and 30's, and the big New Service didn't have that many 45 Colt guns produced either.
Lyman and other mold makers didn't bother to use the new smaller bullet size until the 1950's (when Colt re-introduced the SAA) because cast bullets were sized down in the lube/sizer. And even factory ammo didn't change until the 1950's because having a soft swedged lead bullet a couple thousandths oversize doesn't matter much in a revolver, and they shot much better in the older guns that were by far the majority. Remember S&W made very few guns in 45 Colt before the 1970's when the 25-3 and 25-5 models came out (a FEW Triple Locks, Model of 1950's & Model of 1955's; the run of 200 Model 26's for the Georgia state patrol were the majority of S&W guns). Ruger didn't make any 45 Colt revolvers until late in 1969. So almost all 45 Colt revolvers were Colt SAA and New Service based guns (Model 1909 etc) until the 1970's; and Colt hadn't made very many of them in the new smaller bore size until after they re-introduced the SAA in the 1950's.
Initial changes to the 45 Colt case (rim size was increased) were made in the early 1920's when SAAMI was started. When lever action rifles came out until the 1980's, that caused another change in the case when the "extractor groove" was added just above the rim so that the extractor in the rifles had even more of a rim to grab.
