The answer is the same as if you changed the value in the Marketo UI: depends on the permissions granted to the SFDC API user on the SFDC side. You can test this easily in your instance.
You can also block updates in Marketo from list uploads. That's if you don't want SFDC to always be the master value but are trying to block Marketo-originated updates from specific sources.
To give you a simpler answer, in most cases though, the answer to your question is yes. If you import a list into Marketo with a record where the same email address is already in Marketo, the field values in the list import will overwrite existing values if they are different. And at the next sync cycle Marketo will update the new values to SFDC.
Sanford is correct on the caveats.
2 of 2 people found this helpful
Additionally, we have set up an internal alert in our system which is sent to our team when the first name, or the last name changes for a record in Marketo via list import. Someone from our team manually reviews it to ensure that the new values are in the proper format, and without any typos. This works only if you import not many leads in your MKTO instance. If you regularly import many records, then it will be a lot of manual effort, and it might be better to look for alternate ways to discover such unwanted changes on your records.
You can also restrict the values from updates depending on the quality of the list. If the list is coming from a less credible source, you should mark it as untrusted when importing in Marketo. This will ensure your important fields are not updated via such list import.