Hi Steven,
With Cutover Migration method, it will move mailboxes, mail users, mail contacts, and mail-enabled groups from on-premises Exchange server to Exchange Online. Yes, it will create corresponding mailboxes/mail users/mail contacts/mail-enabled groups in Exchange Online. Therefore, if you already have created them in Exchange Online before Cutover Migration, you might encounter some issues when doing the migration. So, we recommend you delete all the users/mailboxes/groups that you want to migrate from on-premises Exchange server before doing the Cutover Migration.
Moreover, with Cutover Migration, all the mailboxes will be migrated to Exchange Online. If you just want to migrate some of the mailboxes, we recommend you do a Staged Migration. Here are some links for your reference:
Migrate all mailboxes to Exchange Online with a cutover migration: https://technet.microsoft.com/en-us/library/jj874016(v=exchg.150).aspx
Migrate mailboxes to Exchange Online with a staged migration: https://technet.microsoft.com/en-us/library/jj874018(v=exchg.150).aspx
But please note, the Cutover Migration/Staged Migration (the above articles) is applied to the standard on-premises Exchange server. There is no Microsoft document mentioned about how to migrate the mailboxes from SBS 2008 server to Office 365. You can refer to the above articles. However, if you encounter some issues when doing the migration for the SBS Exchange server, we could only provide limitation support about it. Your understanding is highly appreciated.
Best Regards,
Anna