How necessary is it to uodate old tours with new KR Pano version?

That really depends i think.
My question would be for each tour if it is relevant in the first place.
What kind of tours with what functionality?
Obviously it is wise to update them all i guess.
But hard to say what for you is the best option for what tour.

Most often only updating the tour.js file and the plugins should be enough.
But i would check each one, and perhaps re-structure the whole thing..

Tuur