This is a question, not yet the answer, although I'm hopeful the unequivocal answer will be available here soon.

I believe there are several environmental factors that must be correct in order for a spawner to be successfully converted with a spawn egg, and that some of them vary depending on the egg type. My best guess on the environmental contitions are:

1. air or water surround the spawner depending on if the egg is a land or sea animal.
2. the spawn area should be dark, or light depending on the type of animal.
3. the ground under the spawner must sometimes be appropriate to the animal such as mycelium for a mooshroom and grass for sheep.
4. the ground level can sometimes allow open air below the spawner and other times must be directly under the spawner.
5. the egg must be placed at the right time, guessing after it does a spawn poof, not right at the same time the spawner is poofing.

These are my guesses as a starting point for discussion.

Does anyone know once the spawner has been set can a change in environment cause it to revert?

Sure it can, if you got a husk spanner but for some reason the environment changed into another biome which isn’t a desert, it wouldn’t work anymore.

This is also kind of undetailed, I suggest ya’ll to take a look at my guide which goes more in depth for each kind of spawn egg.

here for the guide.

Pretty sure changing a spawner doesn't depend on anything. The environment only dictates whether or not the spawner will spawn, you can change spawners to whatever you want regardless of the blocks or biome around them.
They also shouldn't ever revert unless there's some form of chunk corruption.

I realise people have posted before saying spawners have reverted/failed to convert. I'm confident this is actually because of lag rather than anything else.

Pardon me I must have missed a lot. The last thing I remember regarding this very topic is that staff would no longer support changing spawners since it causesd the economy to break and therefore the map to reset.

@CodyJProductions It's not supported per say, but it is allowed. It's up to the player to figure it out, and it will revert if the player does anything wrong in preparing the environment, i.e. light level, Y level, biome etc.

^ We aren't going to stop you from changing it, but we will also not refund your egg or re-change the spawner for you.

@HaloNest Sure it can, if you got a husk spanner but for some reason the environment changed into another biome which isn’t a desert, it wouldn’t work anymore.

This is also kind of undetailed, I suggest ya’ll to take a look at my guide which goes more in depth for each kind of spawn egg.

here for the guide.

Thanks HaloNest, I saw this when you first posted it, but couldn't find it when I searched. So if we clear the standard 9 x 9 x 3 spawn area around a spider spawned, light it up and place mycelium under it, that should allow for change to a Mooshroom Spawner?

Yep

Apparently Ricky placed his Mooshroom Spawn Egg on a spider spawner, it poofed out one batch of 4 Mooshrooms and reverted.
The area was large enough, well lit, with mycelium filled in flush with the sides of the spawner.
Can anyone suggest what may have gone wrong?

@TimmyBoyT it happens sometimes i have had that happen to me a few times and turn around minutes later and use the same spawner with the same type of egg and not change anything and it worked

Here is the relevant buggy bug report:
https://bugs.mojang.com/browse/MC-95437
Upvote it if you want it fixed...or don't if you don't...

If I understand it correctly, and I'm not sure I do, whether or not that spawner converts successfully depends on if the world saves between changing the spawner and and it spawning its first round of mobs. If it does save the spawner converts successfully, otherwise not.

I'm not sure if this is correct, but if it is you should be able to force this to happen by logging out immediately after converting the spawner and waiting >45 seconds before logging in again. Maybe try it with a couple of spawn eggs you don't want and see if it works.

@NutjobBob Pretty sure changing a spawner doesn't depend on anything. The environment only dictates whether or not the spawner will spawn, you can change spawners to whatever you want regardless of the blocks or biome around them.
They also shouldn't ever revert unless there's some form of chunk corruption.

I realise people have posted before saying spawners have reverted/failed to convert. I'm confident this is actually because of lag rather than anything else.

Yes i also don't think it matters i had a mushroom spawner under an ocean

Just curious, I forget if it was asked or answered before, but does y coord matter? Like would a spawner at y=50 be better than one at y=12 if you were to try converting it to a mooshroom spawner.

@Dennari43
It won't make a difference for most mobs. The only ones the y coord will effect are the ones that need a specific y coord like ocelots, bats, squid and slime.

Nice Christmas hat btw.