Make sure you can own the work for the whole migration. If you lay out the tasks and more than 1 team has to be involved add 10 to the multiple increase over your managers estimates the migration will take by 10 for each extra team.
"But that means if I have a 1 month project and I have to involve 10 other teams it would take 10 years or so"
Yes that is another way of saying it will fail.
If you can figure out how to get up front sign off from all teams so you can just do it all within your own team you will make things go a lot faster.
Separately figure out how to cake slice things. If you have Dev and prod for instance and 10 applications, don't migrate all 10 in Dev first. Migrate 1 app in Dev, then the same app in prod, then go onto the next app. That way wherever you stop at least something will be delivered to the custoner.
"But that means if I have a 1 month project and I have to involve 10 other teams it would take 10 years or so"
Yes that is another way of saying it will fail.
If you can figure out how to get up front sign off from all teams so you can just do it all within your own team you will make things go a lot faster.
Separately figure out how to cake slice things. If you have Dev and prod for instance and 10 applications, don't migrate all 10 in Dev first. Migrate 1 app in Dev, then the same app in prod, then go onto the next app. That way wherever you stop at least something will be delivered to the custoner.