Migrate Client and Deal Data to a New CRM Without Loss or Duplicates

We are switching to a new CRM and need to move our accumulated data across without losing anything or creating duplicates. The database includes several thousand client cards, an active pipeline of deals at various stages, and years of correspondence the team relies on daily for context. A plain export and re-import won't work: the old system has inconsistent formatting, some clients appear twice under slightly different names, deal statuses don't map cleanly onto the new platform, and attachments are stored separately from the messages.

We need someone who has handled this kind of migration before: mapping fields between the two systems, cleaning up obvious duplicates before the transfer, preserving relationships between clients, deals and message threads, and carrying attachments and dates across correctly. After the transfer, we need a reconciliation pass comparing record counts and key fields between source and destination to confirm nothing was dropped, mangled or copied twice.

It would help if you could suggest a rollback approach in case something goes wrong partway through, and leave short documentation of the mapping logic so our team can maintain the database afterward.

What matters most to us:
— zero data loss across clients, deals and messages
— no duplicate records after import
— a documented reconciliation report
— clear communication about risks along the way