Here's the deal. Price reductions are great and allow "older" champions to be cheaper and "new" players to buy them more easily.
However, when it comes to Hextech Loot, especially Champion Shards, the reduction applies as well. Both to "Upgrading" and "Disenchanting" options.
As an example, before Sylas was released, I had a Champ Shard for Quinn (6300 BE - pre-Sylas-release). It was worth the same as any other 6300 BE Champ: 1260 BE Disenchant / 3780 BE Upgrade.
Post Sylas release, Quinn's price was reduced to 4800 BE (960 BE Disenchant / 2880 BE Upgrade).
In my opinion it kind of sucks realising that you just lost 300 BE just because you held on to an item.
I believe the shards should maintain their Disenchant value as of the time they were drawn.