Converting an open-end mortgage to a closed-end mortgage is a refinancing and will require new disclosures.
How you handle the application process is an internal decision as long as the process insures the timing requirement for providing the disclosures are met.
From 1026.40
iii. If the consumer and creditor enter into an agreement during the draw period to repay all or part of the principal balance and the amount of available credit will not be replenished as the principal balance is repaid, the creditor must give closed-end credit disclosures pursuant to subpart C for that new agreement. In such cases, subpart B, including the substantive rules, does not apply to the closed-end credit transaction, although it will continue to apply to any remaining open-end credit available under the plan.
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The opinions expressed are mine and they are not to be taken as legal advice.