What happens, when person sales group is changed?

If person sales group is changed, the person will show up in both sales groups, showing sales that were made in particular sales group.

The exception is when the person was not part of any sales group and was assigned to it later. In this case all the invoices outside sales group, will show up in current (most recent) sales group. 

Same goes for Sales Orders, Quotations and other records that store the Sales group in the record itself. In other cases such as Activities all the measures will be assigned to a single person in the current sales group.