I don't think that there would be a software download just before a flight.
There is a huge amount of testing at lots of levels before any flight software is released - I know, it's kept me in work for twenty years! - most of it is done at the vendor's site but the integration/systems/acceptance is done at the customer's. Many eyes look over each step, independent test and design teams to try to achieve objectivity.
Presumably we are talking about things from a different perspective here.
In the inital testing or when revising any software then there would be software testing/integration/acceptance with the customer which in this case it would be the aircraft manafacturer
Once any software is finalised in the FADEC and delivered to the Airline however it should be relatively plug and play even if you are just replacing the FADEC's for a software revision.