I've gone through such a migration. Huge Go code base distributed across a lot of git repositories had to be moved to a different git host.
It was horrible. A team of people spent weeks. Different projects depended on different versions of the same internal libraries, we had to create a branch for each depended-on commit and make a version of that commit with the new URLs. The expressed goal was to end up with a system that's "exactly the same" as the old, just on a new host. Changing which version of libraries projects depends on would introduce unnecessary risk.
And the result is a code base where bisects are broken and where it's impossible to build an old version of any of the code without a ton of work.
> Different projects depended on different versions of the same internal libraries, we had to...
This is one of the main motivations for using a monorepo with all third party dependencies imported all the way in, and only one version for everything.
Of course it causes a whole bunch of other problems and is kinda expensive to scale, so most places won't do it.
Isn’t this exactly why the Go team recommended vendoring deps for years and years before go modules came up? Even now it takes one simple command to vendor them.