Why? Fastmail always complied with warrants to get your data, so nothing changes because if this law. If you want to hide something, encrypt your e-mails before they reach Fastmail (and do the same regardless of provider you use, why trust anyone?)
I haven't decided myself if I'll switch, but if I do, it's more of a matter of principle. I just don't like how the world is becoming more authoritarian-like, and I feel it will continue to move this way unless people demonstrate their unwillingness to put up with such policies.
I actually thought about switching over this, but I found no service that gives me the same features that FM has (fast web interface, calendar, fido u2f, aliases, send as aliases, custom domains, dmarc SPF and all those acronyms, good support) even if I would be willing to pay whatever (and I was grandfathered into the old $70/2 years plan on FM)
For a family of four, the annual cost would be about half of Fastmail's standard plan.