Yeah you usually can upgrade with Level 2 armour, In the past I have upgraded a couple of jackets that came with cheaper, stiffer armour to D30 L2 as its more comfortable. Approx £25 a pair for elbow/shoulder plus the back, £100 ish to do a jacket.
As mentioned above, I upgraded my mesh jacket armour to D3O L2 elbow, shoulder and back protector. I bought it from eBay, where you can usually find different sizes, for example the Viper back protector comes in 4 or 5 different sizes.
You can upgrade the armour but that is not the same as upgrading a jacket from AA to AAA. The rating is also based on abrasion resistance etc. From the articles I’ve read there is a lot of confusion, for example stretch fabrics are unlikely to meet AAA rating.
