We're retiring Slack because of cost and moving most of the organization to Microsoft Teams, which is already included in our Microsoft 365 licensing. About 237 active users have roughly seven years of Slack history—around seven million messages—including channels, direct messages, group conversations, files, and custom emoji. Former and deactivated accounts would be excluded.
The bulk migration needs to be finished before the Slack renewal on October 9, 2026, so we can move to a much smaller plan instead of renewing the full subscription. Around 30 users will remain on Slack until February or March 2027 and then migrate separately.
Microsoft's native migration tooling appears to cover channels but not the full set of data we need. CloudFuze offers a managed migration service, but the estimate was about $12,000 with a timeline of up to two months.
Has anyone handled a Slack-to-Teams migration at a similar scale? I'm especially interested in whether you used a migration tool or handled it internally, whether you transferred history or made a clean break, and what you would do differently next time.
4 Answers
We evaluated a similar move and also chose not to migrate the historical conversations. A clean break avoids a lot of tooling, mapping, permission, and data-quality problems. Keep Slack read-only long enough for reference or export an archive, communicate the cutoff clearly, and establish retention rules in Teams from day one.
I've helped with smaller Teams migrations, and the clean-break approach was much simpler than trying to reproduce every conversation and file. There may be better tools available now, but for a migration this large I'd first decide what information genuinely needs to be retained, archive the rest, and avoid paying to move years of low-value chat history.
We moved about 100 users from Slack to Teams a few years ago, and the available migration tools weren't good enough for what we needed. We chose a clean break: made the Slack channels read-only, exported the messages for archival purposes, and downgraded to the free Slack tier about a month before renewal. We shut Slack down at renewal. There were plenty of complaints, but the budget savings made the decision worthwhile.
Seven years and seven million messages is probably more history than you actually want to carry forward. A lot of that data is likely outdated, duplicated, or not appropriate as a permanent record. We use a short retention period—around 90 days—and direct people to store anything important in the proper document or records system instead of treating chat as an archive.
That makes sense. We currently tell people that Teams is for transient information and expect messages to be cleared after roughly 48 hours, so a clean break would reinforce that policy.