It's not so much that they can't, but more about it being too much effort to fix the old items. In theory, one could contact a DM who could look at the item and see that it has for example both flaming and frost and the DM could give the player a new festival frost kit which in theory, should be able to be applied to the old item to remove the frost and give it festival frost instead. This should be a theoretical possible fix, but it's probably not worth the time or effort for Turbine. This fix would have to be on a per item basis and require you to prove you need a fix by showing the item. It's slow. It's clunky. But it is possible for some items that have two prefixes, but hard to prove on items that only have frost as a prefix.
Personally, I'm just going to suck it up and apply new festival frost kits to any of my old items in the hopes that it wipes the old frost and fixes the min level. However, I do understand how this could be very annoying to those that put time and effort into these items originally.