How to achieve this:

1. On your Sale.Order create a related field to Opportunity > Stage (stage.id) 

2. Make that field NOT read only, and make sure it is stored 

3. Create an automated action that meets your condition. 

Model: Sales Order (this is important!) 

In this case we are updating any records that were not in Status Sales Order and are NOW in status Sales Order - and we are updating our new related field to the status we want to achieve, which is WON. 

4. Now when you CONFIRM a Sales Order -- you will see your new related field has been updated to the WON status. and your opportunity will update in the pipeline as well.

5. You can now make that field on your Sales Order INVISIBLE so users don't get confused. 

Test again and voila!