In SFDC, you can change the email and the username separately. If you change only the email address, Marketo will not even notice it
If you change the user name, the sync will stop immediately. My recommendation is that you follow this procedure:
- Deactivate the sync in Marketo
- Do you username change in SFDC.
- Collect your new security token from SFDC
- Come back to Marketo, enter the new username and the new token
- Reactivate the sync
Do the whole thing quickly because even when the sync is deactivated, "Sync to SFDC", "Add to SFDC campaign", .. flow steps are still active and are causing errors that you will not be easily able to recover. If you could deactivate the triggered campaign running these flow steps, it would be better, but it might be daunting task.
If the role and profile are unchanged in SFDC, this will not break anything and if the sync interruption is short, Marketo will recover any missing data, excepted for the failed flow steps above.