Migrating existing email into Google Workspace is really two separate problems: moving the historical mail (old messages, folders, attachments) into the new mailboxes, and switching live mail flow over to Google without dropping messages in the process.
Moving historical mail
Existing mail can be imported into the new Workspace mailboxes before the live cutover happens, meaning your team can see their full mail history from day one rather than starting with an empty inbox and a separate archive to check.
The cutover itself
The riskiest moment in any migration is the window between when the old system stops actively receiving mail and the new system starts. Lowering the DNS TTL (time to live) on your MX records ahead of the change means the switch propagates faster and that window shrinks. Cutting over without adjusting TTL first can leave mail split between two systems for hours or, in the worst case, up to 48 hours.
What to check after cutover
After switching MX records, confirm mail is arriving correctly for several different senders (not just internal test emails, which can behave differently), check that no old forwarding rules are still redirecting mail to the previous system, and keep the old mailboxes accessible read-only for a period in case anything was missed.