Bukkit/Bukkit — comment on commit 25f303ccdea8b343f736660508c50af5dff9789d
File: src/main/java/org/bukkit/entity/EntityType.java line 47
By Dinnerbone:

William? And yeah we can; backwards compat! :D These ids are like block IDs, should never change unless we want very messy code handling transitions for the rest of the codebase's life.