Box in knob and tube wiring between ceiling and cellulose?
Code doesn't say that anymore,  at least not in WA and OR.  

Serious study was done on the question  of whether blown insulation on K&amp;T was really having an impact on house fires.  The studies determined it was not.  And so states have been rescinding their laws against blown insulation with K&amp;T. 

Of course AFCI is a magic bullet that intercepts many wiring faults, and I would put it on any K&amp;T.

Sometimes K&amp;T has problems with neutrals being crossed among several hots -- that was Wrong then, just as it is wrong now.  However MWBC is a method where two hots share 1 neutral, and that was legal then, and it's legal now.  The hots simply must be put on opposite poles, and the breakrs handle tied. Regardless, some of the newest AFCIs don't care about shared neutrals, but apparently only from certain makers.

If the house has blown insulation, you might downbreaker one size, merely out of an abundance of caution. 

Other than that, I would continue to use it indefinitely.