I have a client that chose a specific domain on the SAP form to be used as their "authenticated", "send from" domain. We are in the middle of IP Warming, and now the customer is coming back and asking if they can change the domain? To kind of describe this, the client's main domain is (example) abc.com. All of their employees use this domain for email. The domain they entered onto the SAP form is abc-email.com which is not set up for email. The client wants to switch the domain to abc.com. Is this possible or does a new IP address need to be provisioned and IP warming started all over?
IP warming is not an exact science, but a lot of advice you will get is to avoid making a mistake in sending too quickly from a new IP address. If your client's volume is relatively low, SAP on a private IP may actually do more harm than good since your IP address has to earn credibility all on its own, and with low volume, that's very hard to do. If you avoid using SAP and just use the Marketing Cloud's shared IP addresses and domains you might even be able to skip the warm-up since it's not a new IP address and is already a trusted domain, but your reputation will be shared with the larger pool of Marketing Cloud customers and bad tenants will impact your deliverability.
You may also want to avoid using the same top level domain as your corporate email when using marketing cloud, because too many SPF records can dilute your sender reputation. Using a subdomain is a viable option, like mail.abc.com and helps build trust.
When it comes down to what's right for you and for your client, this is a discussion best held with a deliverability expert at Marketing Cloud where you can share more specific information about the company and how it applies to what they're trying to do.