How do unilateral TOS changes like this work in practice? If the previous TOS didn't force binding arbitration, can they unilaterally impose this change on existing users? Basically forcing existing users to "agree" to this? What recourse do existing users have?

I don't use / won't use 23andMe, because of issues like this (the nature of the relationship changing unilaterally). I don't like sharing private data, nothing is more private than my DNA.