The thing is that as long as you take out the no failover active, the other ASA (standby) will claim to be the active and this one (ex-active) as do not have any failover config will be forwarding traffic so you might get havving network problems as both of the firewalls will be claiming to have the same IP address but different MAC addresses.
If you want to follow this path you will need to disable failover on the secondary unit first and then on the primary and make the changes (you will miss the failover functionality but at least you will not cause any network outage).
For me the best way to go is to do it while having the network with failover, make sure you have a backup of the config and a console connection to the firewall so you can inmediatly go back to the previous setup.
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...