I graduated from the Houston PD academy in Feb 1978. At that time you had to carry a revolver for a year and then you could switch to a semi-auto such as a Colt 1911, Smith 39 or 59, Browning HP, etc. Many officers did switch. There were a lot of Colts and Smiths carried by officers when I hit the streets.
After carrying my Smith model 66 for a year I switched to a Colt Series 70, satin/polished nickel finish. I carried that for 11 years and then switched to Glocks, of which I have carried several different models in the years since I switched. At the time I switched to Glocks there was a lot of talk about cops being outgunned and Glocks had just come to America. I miss my Colt 1911 and wish I had kept it but I am a committed Glock fan now (Models 22 Gen4, 23 Gen4, 27, and a new 34 Gen4).
One of the unique things about HPD is that each officer has to furnish his/her handgun. The Department does not issue them to officers. Several years ago the Department switched to all semi-automatics for new officers. There is an approved list of pistols that officers can choose from to carry, mostly .40 caliber, but officers still have to purchase their own weapons. Older weapons could be grandfathered if you qualified with them each year and kept them on your firearms record.
Due to the expense in outfitting a department of about 5500 officers, there never really has been an serious discussion of the department standardizing weapons and issuing them to its officers. Heck, it has taken years and years to agree on changing the uniform design. (And that will not be implemented all at once due to costs.)
Many years ago, when I was a young officer, our police chief was asked by a liberal news person why so many HPD officers carried .357 magnum pistols. The chief's response was, "...those are the officers that can't afford a .44 Magnum" (of which there were many being carried by officers at that time).
You still see some HPD officers carrying revolvers but they are a small minority.