Block replacement

Does anybody know how to replace a block with another block with completely different behaviors without the second block being moveable? My game (seed quest, the url is below) needs a game mechanic that turns some objects into new blocks that can be manipulated in different ways. This would remove the original block, to be replaced with a more useful one. In the game, I have objects that are completely useless, but can be turned into other objects, upon which seeds can be planted (at least, that’s the idea). The closest I can get is with the emit function, but that requires to object to be movable so that it can fall into place. Another option I have is the attacher component, which does not replace the original block, but otherwise gives the desired effect.

from what I understand, you have a normal block and a useful block, and if you do something you can replace the normal block with the useful block.
If that’s the case, use switches, so that normally the blocks don’t do anything but when you do the thing that makes them useful you can turn on the switch so the normal block does what a useful block would do. And for the change you can also use the switch to play an animation that makes it look differently from a normal block.

Man, what am I saying xD