Hi Ben,
I’m not clear about your numbers. Is it now 500-850? (not 500-350, right?)
Anyway, you can try to extend the range in the Footage Tab: Start/End. Go to 500 and add new Tracker-features and track forward. Go to 850, add new trackers and track backward. Then do a new 3D solve.
However: I would not recommend this in general. First of all, I do not know if the lens was not adjusted during the shot or how the movement was. Or even worse, zoomed. Well, it seems to be below one-second start and end. This might not mean a lot in terms of lens or camera change.
Secondly, with the extra data, some trackers might be recognized as less quality than before, but they might be used in some of the Tracker tags. I tested this scenario, and in my case, the quality had changed (I used 150-299 frames first, then, after all, was set up 0-299.) I would have never thought to just expand the time and pad the track. I would always start over, hence why tested it first. Besides, if I have more footage, I track typically much more, to get more data.
Given how much work is typically done after a track, I would spend the short time to really set up the Tracking tags at least new. Check the new solve and its quality very carefully. Which might take as much time than to get a 350 frame shot. Of course, if it was a difficult shot, with lots of manual trackers, that would be a different perspective.
Again, answering tracking questions without having seen the footage seems kind of careless. I hope I made this point clear.
Cheers