grabbed a cave spider spawner in a cart
placed it down changed it to wither skeleton spawner
picked it up
placed it again
it spawned once
and converted back into cave spider spawner
version 1.1 of the modpack

seems to be a comeback of this issue
Creators-of-Create/Create#1401

update it seems to happen regardlef of being picked up
cheating in a spawner and changing it works just fine

Still seems to be an issue in 1.2. Using blaze spawn egg on spawner, changes that spawner's next entity spawned (spawn data), but then it reverts. Should be changing the spawn potentials. This happens for some spawn eggs. Others when applied to a spawner will change it permanently, but retain the previous settings for custom spawners like loot and lighting level, which seems quite odd (ie. magma cube dropping a golden helmet and rusty pickaxe instead of magma cream and only spawning small sizes). Changing a spawner should probably reset it's characteristics to the norms for that mob.

Ellpeck/ActuallyAdditions#162 might have some insight.