Whatsapp is currently testing a feature whereby users can now switch from an ongoing voice call to a video call without dropping the call.
The feature was first reported by Wabetainfo. The report mentioned that there is a now a video call switch button which appears on the screen when making a voice call. The new switch button will replace current chat button which now appears when making a voice call. If you desire to change from a voice call to a video call, you press the button. The other party will then receive a request from WhatsApp, asking if (s)he wants to “convert” the current voice call in a video call.
If accepted, the voice call automatically switches over to video call.
This feature is currently only available to Whatsapp Beta testers. So watch out. It will soon be released to everyone.