There is no function in WeTravel to automatically switch a participant's package.
When a traveler has booked a package (e.g. "Single Room") and then wants to switch to a different package (e.g. "Double Room"), you have to fully refund & cancel the booking. Then, the traveler should book again, selecting the package they want ("Double Room").
Here is how:
1. Go to "My Trips" and click "Manage Trip".
2. Click "Issue Refund" next to the name that you want to issue a refund to
3. Click on "Full Refund" and confirm
Your participant's booking is now cancelled and fully refunded
4. Contact the participant and ask them to book the trip again.
Note: There is no fee for refunds, so no cost for you or the participant.
The participants gets the full amount refunded, including credit card fees.