Hi, this is my latest tool. It helps you to replace specific blocks in your world with other blocks. This can be useful to make for example an iron world by replacing all dirt / grass with iron blocks or if you made a big building with a block, but then there is an update and a new block is added and it fits in much better (like glass and glass pane).
To use this, you need .Net-Framework 4 or Mono. .Net Framework is Windows only, but Mono works with Linux and Mac too.

Usage:

• you choose a world in the list. WARNING: It must be an Anvil world (you know, that new world format)
  
• you choose the ids and the metadata. (Info: to get the ids and data go here: MinecraftInfo)
  
  
• you click Replace Blocks and wait until a message box pops up. (No seriously, it takes much time)
  

License:
By downloading this you agree with the following:
You are allowed to:

• use this tool and everything you create with it
  
  

You aren't allowed to:

• redistribute this software or any part of it
  
• to make money with the software or anything based on it (except the worlds you change with it)
  
  
• decompile it / get the source code
  

I am NOT responsible for any damage done to your computer or anything else.

One last thing: This is based on jaquadro's work.