Marketo accesses SFDC with the API login endpoint, so a change in the login domain that you use would not affect the Sync user. Marketo and SFDC sync should continue to work irrespective what domain SFDC belongs to.
If a change happened to SFDC that affected the sync user, such as its credentials or the SFDC OrgID changed, then Marketo would need to be updated
If there is an issue with your sync after updating the SFDC domain, try the following to resolve the issuce.
If it doesnt resolve the sync issue, contact Marketo support and they will be able to help you out.