If it's an involuntary reroute, and they come with a proposal for rerouting you - why would there be a fare difference? Normally they would either provide you with an option, or try to get you to accept a refund. I can't imagine any scenario in which they come to you saying: we've cancelled your trip, and you have to pay more now if you want to travel.

Unless you are trying to leverage the cancellation (because under what other circumstances would there be any question of an involuntary rerouting) to book something notably different - i.e. on a completely different day, or to a different destination/from a different origin - then the airline should reroute you without penalty.

I can't see why any fare difference would be charged, unless you start introducing your own additional constraints, i.e. you reject their proposal(s) and ask for something that you consider "better".