This last weekend I went through what I thought would be the laborious process of changing my handle from "Tyrisus" to "Kyeotic" on every site I use. This turned out to be breeze.
With the exception of Reddit I was able to change the name on my account everywhere. Reddit does not allow this, and I ended up creating an entirely new account.
Most sites, like Discord make it a button press. Some sites, like GitHub, offer an automated migration process. npm was a bit of a standout with an awkward, manual, email-driven process where I created a new account and they migrated the content and some of my history over.
In all, I spent about 6 hours moving accounts. I spent a lot longer migrating this blog.