World 6-1

The SMA4 version has a taller bonus room. The brick block barrier was raised, creating a clear path to the exit door.

SMA4 has 42 brick blocks here, two less than the original amount of 44.

At the end, the door is closer to the back wall.

World 6-2

The tall brick block wall and moving cloud platforms were moved down, while the fourth wood block was moved up.

After these short brick block walls, the coins and two wood block platforms were moved up.

Leaping over this gap to the ice blocks is easier in SMA4 because they were moved left.

Right after the above comparison, the wood block "L" shape goes lower. The coins were moved left and above a new ice block platform.

Another wood block was added to the platform after the "L."

World 6-2's goal area in SMA4 has a hidden coin that leads up to new ice platforms and two hidden 1-Ups.

World 6-3

This 10-coin block and everything under it was moved down.

World 6-Fortress 1

World 6's first fort has a more interesting beginning in SMA4, complete with added spikes and a shorter jump to the rail lift.

To make it easier to collect this power-up, the rail path flattens out underneath the Question Block.

The spikes at the end of the rail path were moved up to fit on the screen.

In BOOM-BOOM's room, the center blocks were moved closer together.

World 6-6

Where the upper level ends, the Question Block and floor underneath were moved up.

The hills next to the sunken pipe were shortened because the pipe moved up and to the right.

Underwater, one wood block was removed from the "L" shape. The pit after the "L" was shortened and has a wood block on its left edge.

World 6-7

Everything after the first platform was moved up.

After the first two donut lift sets, the ice overhang was lowered to fit on the screen.

The group of donut lift sets leading upward were moved up...

...and on the left is the last set of donut lifts from that group.

The two note blocks next to the end pipe were moved up.

World 6-Fortress 2

World 6's second fortress received extensive changes, most likely to minimize vertical scrolling.

At the beginning, the first jump is a little higher...

...and four big ice blocks were added to the ceiling.

Thwomp number three:

Thwomps three and four:

After Thwomp number four:

Thwomp number five:

An added block helps "big" Mario avoid skidding right into the sixth Thwomp trap.

Between sixth and seventh Thwomp:

The seventh Thwomp:

BOOM-BOOM's room has extra ice blocks for the wall that fills in behind you.

World 6-Fortress 3

The solid wall halfway through has blocks cut out for a new Question Block, which contains a Super Leaf.

The last conveyer belt was lowered, but it was also shortened, resulting in a longer gap behind it. Its Roto-Disc was moved right and almost centered.

World 6-Airship

One slab was removed from the bottom of the second platform.

More wood and a couple vertical bolts beef up the "steps" leading up to the end.

Near the end, one slab was removed from the high platform so that the top would fit on the screen. Also, the rest of the floor change from the above comparison is visible here.