On the 737 MAX, can the pilots switch to the backup MCAS, or switch active MCAS input to another AoA probe during flight?
As far as I understand from reading the B737 Operations Manual and the Lion Air Accident Investigation report, 


Each Flight Control Computer (FCC) receives AOA input only from the onside ADIRU and vane.


  The AOA and Mach inputs are provided to each FCC by the associated Air Data Inertial Reference Unit (ADIRU). Each ADIRU receives AOA information from one of the two resolvers contained within the associated AOA sensor (i.e. the Left ADIRU uses left AOA vane and the Right ADIRU uses the right AOA vane).
  
  Source: FINAL KNKT.18.10.35.04 Aircraft Accident Investigation Report

The Maneuvering Characteristics Augmentation System (MCAS) and the Speed Trim System are controlled by the master FCC, which alternates between A and B per flight.


  Similar to the Speed Trim Function, the MCAS function is also a flight control law contained within each of the two FCCs. MCAS is only active in the master FCC for that flight. At aircraft power-up, the master FCC defaults to the left side FCC; and will then alternate between the left and right FCC by flight.  The master FCC is not affected by the position of the Flight Director switches.
  
  Source: FINAL KNKT.18.10.35.04 Aircraft Accident Investigation Report

This master FCC is different from the master FCC used to compute for the Flight Director (F/D) and the autopilot (A/P). For the purpose of AFCS, the master FCC can be cycled via the F/D switches on the Mode Control Panel (MCP), and the master FCC is illuminated.


  The master FCC is indicated by illumination of the respective master (MA) F/D
  indicator light. The master FCC is determined as follows:
  
  
  with neither A/P engaged in CMD, the FCC for the first F/D turned on is 
  the master
  with one or both A/Ps engaged in CMD, the FCC for the first A/P in CMD 
  is the master FCC, regardless of which F/D is turned on first.
  
  
  Source: Boeing 737-600/-700/-800/-900 Operations Manual



There does not appear to be a way for the crew to know which FCC is actively controlling the MCAS, nor a switch out the controlling computer, apart from pulling the circuit breakers.