You would need to expand the rom and have an expert level of ASM in order to make those modifications. I know it sucks, but because of the way games are programmed, they often do not have the space capacity to allow for changes. It's why alot of older NES games often have minimal graphical representation. Provided there was enough space for extra sprites, yes, I'm sure it could be possible to alter the apperance based on world number.
Try to use SMBDIS while expanding it using Mapper 69 (ROM at $6000), Then make 7-8 copies of the CHR-ROM, And while youre at it, CHR Slots 2 and 3 ($0800 and $0C00 in PPU-RAM) of the mapper will be used if you do that.
At the first page of CHR-ROM, 8080-900F in ROM (0070-0FFF in PPU-RAM) is yours for the copying