Sure, you can refocus any time. Focusing at the beginning or before the start of a mission on any far away object will do in most of the cases...
The only challenge I see is for the scenario of flying towards an object and coming really close (< ~20ft). Refocusing will spoil the footage because of the focus-pumping effect and smooth constant adjustment is not possible.
The only challenge I see is for the scenario of flying towards an object and coming really close (< ~20ft). Refocusing will spoil the footage because of the focus-pumping effect and smooth constant adjustment is not possible.