Validating the posture of your migrated firewall before cut over is biggest challenge, you need to do all that in change window if you are doing a live ASA migration, which increases the risk on network disruption. Here the planning plays a crucial role, like using a spare device or migrating first stand-by if your ASA is in HA, or if a retail kind of deployment than starting with a less crucial branch first etc. For a large and complex migration going with expert services is a better options as those services have well defined process and have build many custom tooling e.g. migrated configuration validation against the original one.
Firepower migration tool allows migration of just Access rules and NAT configuration without really registering the FTD, that could help in some extent in case of live ASA migration by allowing you to review the migrated configuration on FMC without forcing you to re-image ASA. Once you are satisfied with that then you can plan next steps.
The Firepower migration tool allows to migrate from ASA 8.4 version, though it will be better if you first upgrade to ASA 9.1 and take that configuration to migrate. This tool allow to migrate ASA to FTD via FMC only from version 6.2.3 onwards.
This article Migrating ASA to Firepower Threat Defence 2100 - An Example captures an example of steps to be performed before cut over and during cut over. Having a backup of ASA configuration, a testplan to validate your crucial connectivity and the rollback plan is very critical.
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Migration tool 1.0.0 supports migrating configuration on an standalone FTD only, not HA pair. You need to first migrate the configuration on a standalone FTD and later configure the HA Pair. Even if you are going to migrate the ASA HA pair which is handling the traffic you would like to migrate first standby unit.
You mentioned about a tutorial which explains the configuration can be pushed to HA pair, do you mean from Migration tool it can be pushed to HA Pair? Can you please share the same tutorial?
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