Reference: https://www.youtube.com/watch?v=uAb_RldOor8

• Code: this replaces the 145 x 145 from y5 to y100, ~ means relative
  /fill ~ ~ ~ ~145 ~ ~145 command_block{Command:"/fill ~ 5 ~ ~ 100 ~ grass replace",auto:1b} replace
• Idea is replace with command block then each commandblock do its column which is well within the limit of 32768 blocks
• Doing too much will be slow, but the job will be done
• To leave other blocks, the outside replace can be  replace x (eg: replace water / replace air)
  /fill ~ ~ ~ ~145 ~ ~145 command_block{Command:"/fill ~ 5 ~ ~ 100 ~ grass replace",auto:1b} replace water
• Replacing with grass block will end up getting destroyed, do not know if it is a bug or feature.