Want to reset old iPad but not lose data on newest one

Hello,

Sorry if this a dumb post.

I have two iPads - one I bought around 2015 and the latest one around 2020 (9th generation I think)

They are both connected to the same email address although on the older one I have avoided logging in. The older one has no passqcode so I can just turn it on and use a web browser.

The newer one has all the data on it that I want to keep. I want to erase everything on the old one and connect it with a different email address and update the OS.

I am worried that if I enter the Apple ID password on the old one it might muck about with the data on the new one.

Can anyone give me any advice? Thank you.