Update email on old apple ID

I have 2 Apple IDs. One from ages ago that is simply letters, and the more recent one that's a @gmail address. Both IDs are associated with the same @gmail address and as such, it appears that I'm not able to set up Family Sharing between the two IDs. 1. How do I change the email on the really old ID? 2. If successful, will this allow me to set up Family Sharing between the two IDs so that all my purchases are available in one place?

Thanks!