This is going to be a pain if it's recurring.
We decided to treat POs like contracts as they should be. So we know the actual currency before we place a PO, so we don't have this issue anymore.
You can't really create it from the same PO as you may already know.
We used to:
1. Close the PO. Create a new one with the correct currency and link it to the old one for reference.
2. Bill the same PO but leave the currency as is. This has caused as issues with forex gains/losses though. So we just did the one above.