Hmm, don't think you can interchange the inherit legendary/carved shards with the respective primary pieces. Far as I know, 95s are incompatible with 90s and vice versa, as the prime usually determines the type of weapon.
Think of this, if they are able to be interchangeable, then the legendary stable would be better to use than the carved stable due to the balance difference with the former being superior.
The carved verison of the prime and perfect shard is superior to their legendary counter parts.
Carved lightweight offers more crit at the cost of minor speed.
and carved keen offers more crit at the cost of balance.
Additionally, Rank 6 scrolls will be incompatible with 90 weapons/armor as their max rank is R7.
Say that Dullahan's crit cap is 155. That would usually mean that you would need to max out on carved keen and carved lightweight which would total to +15 crit on the current stat of the player and placing the same enchants from the previous 90 weapon (in my case 124).
Assuming that one is Maxed R7 for crit (Immoral + spirited) and has earned the Golden God achievement (134 with spirited, 132 with Judgement without the +2 crit infusions), you can actually reach up to 158 (in the case of white kitty brooch users) -161 critical rating if you have all your compatible spots infused with +2 crit, along with having max rolls on C Keen + C Lightweight shards, and this is without getting the new set rings.
Without the +12 crit from infusions (ring, ring, jewelry, belt, earring, and weapon), you would get around 146-149 critical rating if your current is around the 134 crit area. So yes, it is possible to get crit cap for the new content, just it would be another huge grindfest and a bunch of more RNG walls to deal with.