Of course, you may agree to modify the terms of an existing loan. Use of a modification agreement would be appropriate, and generally would not trigger new disclosure requirements. If you decide to modify the existing loan by having the borrower sign a new note, the transaction would be a refinance, which would trigger new disclosures and other requirements.
There are other options. Instead of a construction to permanent loan you could do separate construction and permanent loans. The permanent loan could be done in the correct amount at origination.