why was betta very stressed after water change?

I did a 50% water change in my betta's 5 gallon, heated, filtered & cycled tank a couple days ago, and I thought we were going to lose him. I didn't do anything very differently from all the other times, so I'm wondering why this time he got so stressed. I noticed it immediately because he sort of floated near the top and his body was leaning to one side a bit, instead of how he usually reacts when he gets back in his tank which is to swim around as if he's relieved to be free of his holding cup. The rest of the day he sat at the bottom or rested on a leaf of his plant or on the suction cup of his heater and just wasn't acting right (not responding to me or swimming around like normal). I half expected him to be dead the next morning, but he wasn't. He ate and seemed to be improving by the time I got home that afternoon, and he seems almost normal today.

For the water changes, I warm the new water to 80 degrees and add Prime 2 drops per gallon since it's RO water that has had 0.25 ammonia in it. I cup my betta in his Petco cup because he's such a snoop and will be in the way of the gravel vacuum if I leave him in the tank. I unplug the heater/filter and siphon out half the water, vacuuming the gravel. I tidy up the gravel or plants that were disturbed and add the new water. By the time the new water gets added it's either at or very close to 78 degrees which is the original temp of the tank. Then I plug the heater/filter back in and put my betta back in.

I tested the pH of the replacement water and it was 7.2/7.4. I didn't test the tank water pH prior to the change but today it tested 7.6 on regular test and 7.4 on high range. Replacement water parameters were all 0 (even the ammonia this time).

This betta is in the 2.5 to 3 year old range, I'm guessing. We got him at petco 2 years ago. I think he may be showing signs of age because there are brownish areas on him and he sometimes seems to have trouble seeing his pellets when I feed him. I'm not sure. I'm just going by some stuff I read about signs of an aging betta.

Would that small of a difference in pH stress him? I didn't acclimate him this time and maybe I should have but I didn't think it was necessary if there weren't big changes in the water. Is it because he's older? He's never reacted this way after a water change before, even without acclimation. I'm wondering how to prevent this from happening next time.