Sales and Purchase Orders can be reopened if credit memos or returns were created if the correct setting are in place. This article will walk you through how to set up the option for this to happen.
Administration > System Initialization > Document Settings
Once Document Setting is open, make sure you are on the Per Document tab. The document field will default to Sales Order. Check the box for Reopen Doc. by Creating Returns/Good Returns/Credit Memos Based on Doc. If you do not want the user to have to confirm the reopening of said document, you can check the Without User Confirmation. Click Update to save the settings.
You can then change the document to Purchase Order and follow the same steps if desired.
Below is a screen shot of the prompts you will receive if you did not check Without User Confirmation.
If you click Yes the respective document will be opened.