I would guess that it's mostly labour. Unless you could get a fleet of identical cars or vans, then every vehicle is going to be a bespoke job, slowing things down.
The same powertrain is also used in their larger models - Master/Movano (can't think of the Nissan one), so that would also be another market. The labour would be minimal compared to the cost of the batteries.
EDIT: esp since you can just use an angle grinder on the old pack
https://fr.wikipedia.org/wiki/Renault_Trafic_III_-_Fiat_Tale...