Update NAST-VSZTP at runtime

I have got a requirement wherein I have to update dispatch time from 3 to 4 for a sales order based on some conditions before saving the order. For example if dispatch time is 3 - 'Send with application own transaction', after clicking save, it should be modified to 4 - 'Send immediately' and output should be processed immediately.

I tried using FM 'RV_MESSAGE_UPDATE_SINGLE' in user exit 'USEREXIT_SAVE_DOCUMENT_PREPARE', but it doesn't processes the output.

Any help would be appreciated. Many thanks!