In large parts yes, but it’s fairly easy to update a plug-in to a GPT (if you’re the developer of said plugin that is)
First, you just need to ask the helper to “update your GPT according to your plugin specifications stored on www.your-website.org/.well-known/ai-plugin.json”. Then, all you need to do is copy and paste your old OpenAPI schema into the new “actions” setup, and you’re done.