I was (a very small, and only part-time) part of a team that did a migration from a Perforce-era VCS to git. It was about 6 weeks of 100% background work -- as in, we prepared cheat sheets, changed or rewrote relevant parts of the CI and deployment chain, while everyone in the company kept using the old VCS.

Then it took about 36 hours to move the relevant code bases (a few dozen repositories, some of them 20+ years old) over to git, including the entire commit history. This was pretty slow. It could have probably be done faster but we figured, since we only do it once, we don't need to invest too much time. We scheduled it on Saturday morning so it would be done by Sunday afternoon and we had some time to test it before Monday hit the earliest timezones.

We had a backup plan in place in case it didn't work but it wasn't necessary.

The two repos were automatically synced for a while, to allow people who had started work on something on, say, Friday morning, to commit their changes to the old repos, rather than have to replay their work on the new ones. After that the old repos remained read-only, but we preserved them because, obviously, our issue tracker had twenty years' worth of references to old VCS revision numbers.

So it took about 6 weeks of indulgently part-time work (our infra team worked several hours a day on this, I worked... about an hour or so a week?) to plot this daring plan, and about two days to make it happen. The total amount of work disruption was likely non-zero but certainly small enough that most people didn't complain of anything other than having to learn git.

I can't say about Bamboo to CircleCI but, having gone through one of these, I can say with 100% confidence that if you start out right -- that is, your existing repository is not a mess of stale branches and weird merges -- and you don't go about moving fast and breaking things, moving from even an ancient revision control system to git is not really apocalyptic.

Not if there is an open protocol with multiple conforming implementations.

Really? Was it easy enough to define mappings between all the various fields/ticket types etc? I guess you could do it per-team across an organisation.

It always seems hard enough to get information out of JIRA using JIRA when I watch our PO in planning meetings.

Be curious what the size of the engineering team and company was.

Asana is great for SMBs but for larger developer teams it lacks a lot of features and integrations.