Milton,
Changing a Purchase order does not change Sales confirmations. V_V2 can change sales confirmations.
You can exclude existing confirmed sales docs from V_V2 as mentioned above. In general, such docs will thereafter not be touched by V_V2; so they would not be in danger of losing an existing confirmation. Whatever confirmations they had previously would be preserved.
For some exciting reading on this subject, enter V_V2 and hit the blue information button.
For more exciting reading on this subject, try http://service.sap.com/sap/support/notes/1861066
Best Regard,
DB49