You will not reassign the account. Marketo can only assign leads and contacts.
What you can do is use the "MSFT Type" field that, when empty, tells you the lead has never been pushed to MS Dynamics.
Thanks for the input Greg, I dont think I made myself clear though.
All our Marketo leads come over from Dynamics as contacts, the vast majority of these are preexisting CRM contacts that have the Account Owner set to one of our CRM users / sales people. These leads are then marketed to in Marketo, once they're through the lead lifecycle then they are synced back to Dynamics, but in the Sync to Microsoft Flow step it requires that we set the Lead's Account Owner field with as CRM user field. We would like to not overwrite that Lead Account Owner field.
Hope this makes sense.
If the lead is already in CRM and assigned an owner, attempting to sync the lead to CRM will cause a failed sync (and the owner will not be overwritten). Also, Marketo will not overwrite the current owner if it's already populated in CRM. The only way to sync a lead (as a MS lead) to CRM when it's already a MS contact in CRM, is to create a purposeful duplicate in Marketo and hide the original lead (MS contact) in a hidden lead partition.