Quick Answer:
If You go to your Sales & Receivables Setup, and set the 'Invoice Nos.' and 'Posted Invoice Nos.' / 'Credit Memo Nos.' and 'Posted Credit Memo Nos.' to the same number series, the 'unposted' no will carry to the posted document.

Longer Answer - In NAV - and now BC, the Invoice and Credit memo is a draft, whereas the posted document is - well - posted. In the posted Document, Most companies do not want to have 'gaps' in the posted document range. By having two different number series, you ensure that the posted document are serialized in the order they were posted.
If you are OK with them not being sequenced that way, you can specify the same no. series for both the unposted and posted. However doing that, will insert a Posted Record for any 'draft' invoices and credit memos you end up deleting.
Note - Some users will also use the same no. series for both Invoices and Credit memos - which can make some sense as well - all depending on your scenario.