I haven't heard of this specific case, however, your plan needs to be more detailed.
If you are running SFDC+Marketo live you will still have valuable data coming into the system while you try to migrate over to MSD. You will also have to migrate things like pages, workflows, etc.
In the other cases I've heard, firms usually end up running both systems for a month or two until everything is migrated properly. Then they shut down the old system. Of course, that's when they migrate from Eloqua to Marketo and can have both systems connected to SFDC.
Since you are migrating SFDC to MSD, there may be some data issues. I'd consider setting up MSD first to get all the workflows, pages, forms, etc ready with a separate Marketo+MSD instance. Marketo should be able to help you do this. Then, over a weekend, shut down Marketo1, migrate all SFDC data to MSD and go fully live.
But that's still a rough overview. I'm sure your migration consultant, Marketo, and MS can help in more detail.