@Ollo-Heß
I understand the desire for it to be simple for the rectangular column selection case, but it is important that you understand why it is not so, and thus why there is no simple 2-step get-selection and replace-selection for this.
Let me first ask you some questions about what you are doing:
Does the initially selected rectangular block ever have any virtual space in it?
Are all of the lines/rows in the rectangular block you are replacing with of the same length (same number of columns for each line; note that it is fine if this is different from the number of columns in the initial selection)?
Does the rectangular block you are replacing with always have the same number of lines/rows as the initially selected rectangular block?
[Maybe these questions will get you thinking along the lines of why this can be a difficult thing to do…]
If the answers are “no”/“yes”/“yes” then the situation is “fairly” simple, and I can describes some possible ways to implement (but let’s have those answers first). We can of course discuss the cases where the answers aren’t “no”/“yes”/“yes”, but the complexity increases. :-)