Hi everyone! I agree we haven't spelled this out as clearly as we should have. We did cover this in the preseason FAQ.
The TLDR is that ALL shard disenchant values are being changed from 33% of value to 20% of value. So if you have shards lying around that you're not intending to use, we recommend disenchanting them before preseason kicks off on November 8th to get the most possible essence out of them.
More context below:
Are you making any changes to the way disenchanting shards works?
Since you'll be getting more shards thanks to the new leveling system (and with other recent changes to honor and events), we'll tune down the relative value of the essence you get from disenchanting shards of all types—champ shards, skin shards, and ward skin shards.
In the current system, for example, disenchanting a champ shard will get you about a third of the BE you need to buy a champ at that shard's tier. In the new system it'll get you 20% of the way there. Also, with so many more champ shards in the system, we need to remove the ability to reroll them into permanent champs—but we’ll occasionally sell mystery champions for blue essence. For these reasons, you should go ahead and reroll or disenchant any shards you don't want to upgrade before preseason hits so you can get the most essence possible out of them.
To make up for essence drop rate changes, we'll tune the way loot boxes work to increase the amount of skins you'll get from them. We'll also add "bad luck protection," so you won't ever go on long streaks without getting a skin shard or a gemstone from chests.