I spoke with AGV in Costa Mesa, CA. The GP-tech has 3 different outer shell sizes and 4 different internal EPS sizes. To swap different padding/liner size, the outer shell size does not matter. As long as the internal EPS size is the same, you can swap the padding/liner. Here are the sizes that share the same internal EPS size. XXS/XS, S/M, L/XL, XXL/XXXL.
S and M does share the same internal EPS size. So if you have a S or M helmet, you can interchange S and M padding.
I just got my hands on a limited edition Rossi Dreamtime GP-Tech helmet in a size S. I wear a size M. I just wanted to make sure I can put size M padding in a size S helmet