At the moment the WHMCS to WHMCS Import Script does not migrate the Notes in the Clients Area when merging data from one installation to another.
It would be beneficial to have this data moved over as the Notes can potentially be imperative to the day-to-day operation of a WHMCS installation, and it would ease the hassle of having to manually enter in any missing Notes in the Clients Profile.
This feature request was submitted on the behalf of a customer. If this is a feature that you would like to see possibly be included in a future version of the product please don't hesitate to comment and vote.
Thanks for sharing this idea. I'm pleased to advise that client notes are one of the items now merged in the new Import Assist script: https://marketplace.whmcs.com/product/46