If the clients who already holds a pre-approval have found the property and are ready to take the application to formal, please follow the below steps:
Check with banks to know their process to proceed to formal (some banks will require an email with supporting documents whereas other might require you to amend AOL itself).
Order valuation with the bank and do a new pricing as per the new loan required (as clients might not require exact same loan as they were pre-approved)
Keep the settlement date as per contract of sale in the AOL.
Do the process as advised by bank to proceed from AIP to formal approval.