well, you're not rebuilding the turret, so there's a major portion of the cost gone already. the engine is a large investment, but those would be due for swapping soon anyways. in an ideal world, half a year of design and prototyping, a year of field trials, and balancing the suspension for the new weight distribution. but in essence, with the europowerpack you are removing over half a ton of weight, and redistributing it. and you're adding an engine that's already in use in the UAE leclerc so it's been thoroughly tested.
if you were adding weight then yes, you'd have a lot of new stresses and things that could break, which would require quite a lot of strengthening and testing.
anyways, add 2-5 years of infighting and bureacracy, and you're still not close to the 15-20 years or so it took to field the Puma IFV.
and IMO the leopard is fairly future proof. add an unmanned turret, and you essentially have a good competitor for the armata.