Does impedance of a speaker change when one bi-amps?

Assuming the crossover is split, yes.

So in a typical two way the tweeter impedance will go to infinity in the bass,
and the opposite in the woofer. The impedance will rise to infinity in the treble.

Of course, that "infinity" may be at 1 Hz and 1 MHz, so in the actual audible range you'll never hit it, but 30-100 Ohms maximums are not impossible.

If you want to play with this, using a crossover simulator like XSim and designing a very basic crossover will help you see what is going on.

Best,
E

Forgot I have a blog post for this:

https://speakermakersjourney.blogspot.com/2016/12/crossover-basics-impedance.html


Best,E

Thank you.