What if the layout is changed for a record assigned to a sales rep? Can the user continue to access it even though he/she does not have access to the new layout?

What if the layout is changed for a record assigned to a sales rep? Can the user continue to access it even though he/she does not have access to the new layout?

Yes. The owner of a record can continue to access and edit a record even if the layout of the record has been changed.