How much does your core change over a campaign?

[Imp]

I play just as a commander quite often progress through formations rather than ranks though. Might start in charge of foot sloggers depending on side playing few tanks if any though might have the odd TD but mainly ATGs. Then might upgrade to a mech formation with a bit of armour before switching to an armoured unit. Easier to do the other way once have some experience mind you but gives diffrent challenges. Recently started buying 2/3rds as second line units & upgrading to regulars once become elite, can be hard at start depending on side playing. Nowadays just upgrade when I need to not as soon as new gear is out & always replace the whole company not piecemeal. I often delete & rebuy so formation is correct for the time but thats just me lose your experience doing that.
Much as dont upgrade straight off if strugling might do combat trials & buy a platoon of new gear with support points in the mean time. If apprporiate will change 1/3rd or so of my infantry to ski & back for winter if short on transport.