Thijsie,
When you say replacing blocks in AutoCAD, I take it you mean maybe placing an updated block in your drawing. I can't remember about the older versions, but when using 2002, if a block is in the drawing already and I go to insert it again, I am prompted as to whether I want to replace the existing block.
Maybe I am misled by your question. What version are you using?
Haggis.