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.
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.