I'm trying to change the mob on a spawner and every time I try it gives the message "An internal error occured while attempting to perform this command."

Can anybody help me with this? Thanks.

 

Did you try to change a dungeon spawner or a spawner bought from the shop?  Since the dungeon spawners have been known to be a bit iffy.

It was a blaze spawner in the nether.

Yeah, natural spawners don't like the spawner command very much, try to save and buy your own spawner and the command should work fine.