Anyone know of a way to do this? Or at least how to disable automatic switching? Cheers!

Other than flying indoors -- which I recommend against -- it isn't possible. ATTI and OPTI modes require GPS positioning to be disabled, but the Mavic doesn't provide a way of doing that, at least not through software. It's possible to block GPS reception using aluminum foil, but that's not something most people are interested in doing, so the short answer to your question is that it isn't possible to perform the switching yourself or control when the aircraft does so on its own.

DroneFlying Posted at 2017-6-27 10:58
Other than flying indoors -- which I recommend against -- it isn't possible. ATTI and OPTI modes require GPS positioning to be disabled, but the Mavic doesn't provide a way of doing that, at least not through software. It's possible to block GPS reception using aluminum foil, but that's not something most people are interested in doing, so the short answer to your question is that it isn't possible to perform the switching yourself or control when the aircraft does so on its own.

Darn... Thanks!

And that question is still a claim from experience pilots.
Cheers.

If you stick a GoPro on,  it will disable your GPS.  Just saying...