I am in the planning stage to upgrade ASA 5510s failover (active/standby) pair to 5540s. Is it possible to load the config on the new 5540, replace the secondary 5510 with it, fail over to the 5540 and then replace the primary 5510 with the other 5540?
Unfortunately this is not possible. Failover requires that the hardware of both units is identical. As a result, the migration will require some downtime while you swap out the hardware.
Hope that helps.