We recently had a change in our main general inbox address. We have updated this everywhere but Marketo at this point, but the both addresses are still active.
If I changes the settings for the default From Email in the Admin section, will this have any negative impact on deliverability?
The default From Email is only used in situations where the From: line in an email is empty, so changing the default should not impact your deliverability if you are putting a value in the From: line of your emails to begin with.
Did you just change the XYZ@yourcompany.com or did you also change the domain name? If you also changed the domain name, remember to create the needed txt-records on the new domain name with your Marketo instance's SPF and DKIM info.