Hi @Jake_Cashion, welcome to the forum!
It seems you created new accounts for the new email addresses. If you wish to keep having access to any information in the old accounts, you can suggest to your colleagues to merge their old accounts with the new accounts (the ones associated with the new domain).
If you don’t need access to the old accounts, you can remove those accounts from your Organization following these steps so they don’t appear as suggested when you assign tasks.
In regards to your last question, the account owner will be able to update the name accessing the Profile tab in “My profile Settings”.
I hope this helps! Let me know if you have any questions!