Using Custom ship designs, in place of default ones.

How would one replace the vanilla games ship models, with all custom ones, w/o having to re-create from the ground up as the game progresses. I would like the games pre-made configurations to be used, but with d/l ship models. If this is doable, I would like it so the default models never appear again as the techs become available, but rather, the same specs, different models.