Rerolling for structural design

[glawen]

So, I'm a beginner (and I find the game to be amazing), and I've just designed my first tank model. Unfortunately, its structural design stat is a measly 83.
I'd very much like to reroll it, because designing a new model from scratch would cost me the ungodly sum of 800 BPs (I get 250 BPs a turn to be divided among my 5 existing councils)

When I load my former turn savegame and end the turn, no matter what I do, I get the same (crappy) structural design stat (and all other - bad - stats are also the same).

My question is simple : when is the structural design stat for a model rolled ? Is it (as I unfortunately suspect) when you take the design decision, before spending all the necessary turns (and BPs) to create it ?