Multiple apple id's and how to merge them across multiple personal devices.

I originally had just an ipod and a laptop to sync my itunes music to. Later, I bought an ipadPRO and was able to synch music on all devices. I lost my ipod for awhile, and during that time had to change my sign-in information on the other devices (for itunes and icloud). I found my ipod now but cannot sign into itunes with it as my id has changed. I tried several different ways to update my ipod to reflect my updated username and password, but it continually tells me that, 'this is the incorrect email or password', as it still has my old username and password in it, which i can no longer access.

How do i 'match up' my laptop and ipad to the old ipod so that I can have the same username and password? That way, I can sign into itunes music on the ipod and update my music library?