Tip: Take advantage of easy to install Patches in your hack. The No More Sprite Tile Limits or SA-1 patches should be considered if you're encountering glitches where Mario and/or sprites are turning invisible.
Similar to Smallhacker's Separate Luigi GFX patch, except it will easily let you use any RAM address at any value to use as catalyst to change the GFX. This means you can easily change Mario's GFX mid level for a power-up or whatever you want to give him. Also is compatible with MarioExGfx, though it won't let you save $7E2000-$7E7D00 for free RAM if you do use it.
Also: With a little bit of work, you can use as many different GFX files as your ROM can hold, so you can have a lot of different characters if you need. (explained in the patch)