I recently upgraded from the Free plan to the Plus plan. Since the upgrade, I can no longer upload files in threads that I started before upgrading. However, file uploads work in newly created threads.
Any assistance on resolving this issue would be greatly appreciated as I rely on this feature for ongoing conversations.
Thank you in advance
No solutions for you, but wanted to let you know that I’m experiencing the exact same problem. I contacted support on 12/23 and got a quick response with some things to try. None of them worked and i reported back to support. Still waiting on a response. Did your problem get resolved ?
I got a response from support that helped me fix the problem: You need to change the model for your migrated chats from “Chat GPT 4o Mini” to “Chat GPT 4o”. The mini version does not have the ability to attach files. GPT 4o does. It looks like 4o mini gets assigned to all chats created under the free plan after upgrading thereby disabling file attach. Manually changing the model fixes this. 
This worked!
Appreciate the support.
Thank you
Dear OpenAI Support Team,
I am writing to you regarding a recent issue I encountered while attempting to upgrade my account to Chat GPT Plus. I paid 20€ and my account was not upgraded to Chat GPT Plus. Despite my attempts to contact your team through both email and chat, I have not received any response.
As a customer of OpenAI, I am extremely disappointed with the lack of response and attention given to my situation. I would appreciate it if you could investigate this issue as soon as possible and provide me with a solution.
I have already paid for this service, and I expect to have access to it.
I hope to receive a prompt response from your team and a swift resolution to this issue. Thank you for your attention to this matter.
Best regards.