Switching engine, reulations & law?

[polo-sib]

Say I had two polos, such as the following:

Mark 4 Polo 1.0L 5Dr
Mark 4 Polo 1.4L 3Dr

And I wanted to switch everything from the 3door into the 5door and vice versa so I'd end up with:

Mark 4 Polo 1.4L 5Dr
Mark 4 Polo 1.0L 3Dr

What would I do to make this change official, to VW so they now know the number plates have different engines and so does the government, obviously I'd have to tell insurers, is there anyone else I would have to contact and when contacting what should I be looking for?

As you see a lot of engine mods/switches, yet no details of them letting a 3rd party know of the change, so I was wondering if there are any laws & regulations towards this as well?