This doesn't seem to be the case for me. I have access to GPT-5 via chatgpt, and I can also use GPT-4o. All my chat history opens with the originally used model as well.
I'm not saying it's not happening - but perhaps the rollout didn't happen as expected.
I have Pro. To get the old models, log into the website (not the app) and go to Settings / General / Show Legacy Models. (This will not, as of now, make these models show up in the app. Maybe they will add support for this later.) (Also, 4.5 is responding too quickly and--while I am not sure this wasn't the case before--is claiming to be "based on GPT-4o-mini".)
(As a follow up, at some point the model selector on my phone also got the legacy submenu, and this is definitely the actual ChatGPT 4.5. I just started flipping back and forth between 5 and 4.5 on the same prompt, and it really makes me wish they would let me keep paying more for 4.5 usage.)
I'm not saying it's not happening - but perhaps the rollout didn't happen as expected.