Honestly the new sprites aren't bad, and the improvement is very visible (in the same way the sprites in Layer 2 were to layer 1 even without the proportions change). The way Dia's face is drawn make her look a lot older, I always assumed she was maybe a bit older than Adventure Kid. Basically I thought she was 16 with anime colored white hair, but now she looks in her mid 40 with white hair due to old age.
My biggest issue with the change is how the overall pixel density is affected. The previous character sprites and the cards were on a similar level of details, but now the portraits look more dense. Basically you have the overworld sprite (Chibi), the cards and the portraits each on a different level of detail whereas the previous character sprites looked more in line with the cards, and the proportions reflected the way the overworld sprite look so everything was more cohesive.
I don't think the proportion change is actually the issue, as the guard doesn't look weird at all to me. I believe it's the way you shade the hair with a lot of small lines instead of bigger masses of colors, and some of the details like the stray hair. If you take the character sprites in a vaccuum, the faces are more pixel dense than the rest of the body. Perhaps the guard isn't as dissonant because he's got no face details at all.