We migrated from awbs to whmcs some years ago, it was a time consuming and costly process. Took us 6 months to migrate.
We wrote our scripts ourselves, did not use anything from whmcs. Our strategy was to use the WHMCS API as much as possible to do the migration, http://docs.whmcs.com/API. Fetch data from awbs db and feed it to the api to create customers, services etc. Most was possible to do through the API, some needed coding directly to the db, but not much. Some we created manually, the products for example. I believe using the whmcs API is the most robust way to do it.