ok seems I mis-understood the changes.
If for example you trade you Sauros in to Xur, you gain a new Sauro, yes you have to level it from the start, but the base damage starts at 300, with 323 as the max.
In short you won't lose raw damage just the mods off the gun until you level it again, so not a bad trade off, also Exo's don't require assendant materials anymore, just an Exo shard for the last node.
Exo shards cost 7 coin from Xur, not sure on the charge for a new tier gun, also I wouldn't bother putting any xp into any Exo gears before the patch now, as all your doing is wasting materials, if it's a base gun or fully upgraded weapon you can trade it in.