Since you are upgrading from 7.2(1) to 7.2(3), you should have zero downtime. But, I would always check and make sure that your failover is working properly with 7.2(1) before you bring in another variable into the equation, which is 7.2(3) :-)
Please refer the section "Performing Zero Downtime Upgrades for Failover Pairs" in the below URL.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...