Laying vinyl planks over existing planks

When we built our house 4 years ago we had vinyl planks laid but we would like to change them to a darker colour. Can you re-lay vinyl planks over an existing vinyl planked floor? or do the old ones need to be ripped up and the floor prepped?

They will have to come up. If you tried laying over them you'll have nothing but problems, The biggest being the glue would not adhere properly.
Unfortunately you will have to rip them up, and you will have to get the old glue sanded off the floor.

You COULD buy ALLURE.

Allure is a PVC FLOATING FLOOR.

It is 100% waterproof PVC, just like you currently have, but floats over your old floor, without being glued down, just like engineered timber or laminate.

You can buy ALLURE from Harvey Norman in all states except WA and NT (unless you want to freight it from the east coast).

There may be other floating PVC vinyl floors on the market, but I haven't heard of any yet.

In the States, they call it Trafficmaster Allure. If you google it, you will see what I am saying.

The tiles must come up, but they are various floor leveling compounds that can be applied to the adhesive to get a good base for the new planks. Removal of the old adhesive is extremely difficult. I assume it is a concrete floor.