I have never found an official source on that question, but i have changed the app domain several times.
Answer to question 1:
As you already mentioned, you cannot configure two App-Domains in SharePoint. You have to change it. This is a very straightforward process:
- Configure the new AppDomain in DNS and create a wildcard-record as described here
- If you have SSL-Encryption enabled, you need a wildcard-certificate for the new domain
- Change AppDomain in SharePoint CentralAdministration (CA -> Apps -> Configure App URLs). You should keep the AppPrefix unchanged, as this would require additional steps.
- If SSL: Change the certificate on the related binding in IIS
- Perform an IISReset and test your Apps
- Delete the old AppDomain in DNS
Answer to question 2:
Normally an App does not contain any hardcoded URLs, as the first part of the AppDomain gets randomly generated at first deployment. So there is no impact on changing AppDomain if Developers follow the guidelines.
As always: Try this in a testfarm, if you have a complex environment with lots of apps. Maybe some developers use hardcoded AppURLs, even if they shouldn't.