I really love this approach. Many services which increase their subscription prices also do so for existing customers.
To play devils advocate here, Protons costs are not fixed. The storage, compute, and people behind your email (and other services) continue to cost more today than they did when you signed up. I would also expect, as a business user, that your prices to customers have increased at some point to cover higher costs of supplies and needed overhead. It seems fair for Proton to do the same and I wouldn’t expect this to be how it always works (and in fact I’d expect at some point in the future you’ll get hit with a catchup increase, either because you’ll need to change your billing cycle or Proton just can’t continue to deliver the service for what you pay on your grandfathered rate). As long as rate increases are not egregious and are well communicated by companies I respect, I see them as an unfortunate but fair way to keep those companies in business.