Use new primary line for new text messages and existing threads for dual sim lines on iPhone 12 Pro

I am using both a primary (new number) and secondary line (old #) on my iPhone Pro 12. I am trying to transition all of my texts to start using my primary line, but when starting a new text or responding to an existing thread, it defaults to the previous secondary line. Is there a way so I can have all conversations default to my primary number? I did set my messages to have my primary set to default.