Is it possible to change lawyers midterm (a few months before trial is scheduled)? If it's possible, is it feasible?
"Possible" and "feasible" are synonyms, so I'm not sure what the second question means. Yes, it's possible/feasible.
Do things start all over or does the new lawyer just review what's been done and pick up, continue on, from there (e.g., lay depositions have been done but not expert depositions)?
No, things do not start all over. Under certain circumstances, a new lawyer may be able to get a few things continued to allow him/her to become familiar with the case. Otherwise, the new lawyer jumps in right where the former lawyer left off.
does it make more sense to bring in a second, having two attorneys from different firms
Only one familiar with the case can comment intelligently on this. It may be that bringing in a new lawyer and completely ditching the current lawyer won't be practical because of the complexity of the case. If you keep your current lawyer and simply add a new lawyer, you need to have clear discussions with your counsel about this and about your clear expectations in terms of no duplication of work.