Except for the new episode, E1M1 is the only level that changed in Ultimate Doom. The changes are:
1. There's a new switch on the pillar to the right of the staircase which leads to the green armor. It's on the north side of the pillar. It opens a door leading to the central area with the blue armor in the acid pit. Walking through this door opens another door which allows you to enter the central area from the room to the north (the room behind the first door).
2. There are some new items in multiplayer mode. In the northwest corner of the starting room is a shotgun behind some barrels. And south of the staircase which leads to the green armor is a berserk pack (skills 1-3 only).