Prep: build the destination before you rehearse the journey
A rehearsal only proves something if the destination looks like production. So the Cloud site comes first, built out properly: apps installed, SSO through Atlassian Guard, domains verified, SCIM provisioning connected. Identity deserves to be called what it is — a full workstream, not a line on a checklist. Jira Cloud keys every account on a unique email address, and migrated data attaches to any account that already exists at that address; duplicate emails will not migrate at all. Directories synced from your IdP before the first run are a precondition, not a nicety.
Alongside the identity work, Atlassian's pre-migration checklist gets worked line by line, not skimmed: firewall allowlisting for Atlassian's Cloud domains, admin permissions in place, instance sizing checked, and backups taken on both sides before anything moves. None of it is glamorous. All of it decides whether your first test run measures the migration or measures your unfinished prep.
Two full runs into sandbox, then production
One test migration is not a rehearsal — it is a discovery exercise. The pattern that holds dates is a complete test run into a sandbox — sandboxes come with Premium and Enterprise plans, and Atlassian's free Cloud migration trial can fund the rehearsal environment at your current user tier — then validation, then fixes, then a second full run. The first run tells you what breaks; only the second tells you whether you fixed it. Production comes after that, not before.
One mechanical detail catches teams out: JCMA never overwrites data that already exists in Cloud. Every repeat cycle therefore needs the sandbox reset, or the previously migrated projects deleted, before the next run. Build that reset time into the schedule — otherwise the second run slips, and it takes the cutover date with it.
Validate what breaks quietly — and time everything
Validation is not eyeballing the issue count. It works through the layers that fail silently: workflows — above all post-functions that depended on apps — permissions, boards, filters, issue links, and app data wherever a vendor path carried it across. UAT is deliberately mixed: power users, who find the sharp edges, alongside occasional users, who find the confusing ones. Every point of confusion is recorded, because that log is the training plan.
And every run is timed. Atlassian publishes a rough JCMA throughput — about five million issues per 24 hours — but no formula for any specific run, and it explicitly declines to estimate downtime. Your rehearsal is the only benchmark that counts. Its measured durations go straight into the cutover runbook, next to named owners and steps, which is why a rehearsed migration asks its change board for a window it has already demonstrated — not one it hopes for.
The Rehearse checklist
- Confirm the directory cleanup from Assess has landed — no duplicate emails, no group-name clashes — and re-sync the IdP before the first test run
- Stand up the Cloud site in full — apps, SSO via Atlassian Guard, verified domains, SCIM — so the rehearsal targets a production-like destination
- Run at least two full test migrations into sandbox, resetting migrated data between cycles (JCMA never overwrites)
- Validate workflows and app-dependent post-functions, permissions, boards, filters, links and app data after every run
- Run UAT with a deliberate mix of power users and occasional users, and feed the confusion log into training
- Record measured durations from each run and finalise the runbook with owners, steps and timings
Next step: Cut over
What rehearsal leaves behind is a runbook that has already been executed twice — owners named, steps sequenced, durations measured rather than guessed. The final step, Cut over, executes it for real: users, groups and attachments moved ahead of the window, the source set read-only, the remaining delta migrated and verified inside the agreed change window before anyone is invited in.
This step is where the homepage FAQ answers on migrating without breaking change control and on getting a defensible, phased plan are actually earned — rehearsal produces the tested runbook and evidence both answers rely on.