Thank you @MoebiusL for sharing the required hardware change. This was my first SMD soldering too, but it was not too hard to do. As illustrated in your picture I took one of the Zero-ohm-links supplied with the nodes (a white strip of about 5) and soldered it over the two JP15 pads, functionally similar to a through-hole pin jumper.
I added a bit of flux on the two adjoining pads, heated them up with the soldering iron and lightly touched each of them with 0.3 mm flux cored solder, still applying heat. I carefully placed the small jumper over both of them using a pair of pincers and applied heat to solder it in place. As I did not have a suitable SMD tip for my iron, heat spread over both pads. Actually this helped in this case as the jumper settled quite nice. Finally I measured resistance over both adjoining pads to verify they were connected. I then changed the pin mapping for lmic.
My Whisper Nodes have been posting data to TTN for a few days now, and it seems to have resolved the issue in a reliable manner. I assume this was by design and part of the reason why the jumpers are supplied. So far I love these nodes and I think they are perfect for low energy applications.
I have one related question - if I want to change the u.FL connector for a SMA edge connector, do I have to desolder the u.FL connector? I assume I do, but I would like confirmation from someone who knows @wisen .