When I made the switch from @Gmail to my own email domains hosted by Fastmail, I never had any problem changing the email on my 100+ accounts with various services.

That was about five years ago, I imagine it might have been more difficult in the past, say 10+ years ago.

Most of the sites I have accounts on did let me change my email. Looking at my password manager, about 1/5 of those that use email for login don't use my current email.

I wouldn't say it is easier than expected though, logging in, changing your email, confirming the change, and then making sure the password manager was updated was a bit of work.

Can you cite an example of a big, well-known, etc service that enforces a 'you can't change your email address' rule? I haven't really seen this sort of thing in the wild to be honest. Maybe one or two over 20+ years? I think the norm is using a username or an internal identification number (such as an UUID). It is rare the email address is actually used as the primary key to identify an account.

I was able to change it on every single service I use except for a well known Canadian pizza chain.

Oh yeah. I did this in 2021. I got the list out of 1Pass and just did them in small batches. I hit a few that wouldn’t let you change but nothing important.

I had all the important stuff done in a week.

I didn't keep exact numbers, but of the ~400 Hide My Email aliases I have, probably 85% of them allowed changing email in the online self-serve, 10% required contacting customer support, and maybe 5% needed a new account.

The majority of mine are changeable, but the process is typically convoluted. The two that I have run into so far that I can't change are OpenRouter and Ecobee.