Maintain Supplier Information for an Item

Good Afternoon

Question regarding the impact of updating Item and Supplier relation when there are open transactions in the system. 

If I change the supplier site on an item, what would the impact be on any open transaction for that item? 

Also if I added a new supplier site to that item what would the impact be on the open transactions (open purchase order, receipting etc) if any?  

Thanks in advance

Emilton