The prices rose around 20% and only for the more expense ones. Apparently that was enough to stop the sales immediately.
It actually turned out that the change caused the EV sales all but stop completely (quite to the surprise of the tax authorities). So they're looking at re-exempting a few thousand cars every month.
Fun fact: The danish nickname for the tax authorities is "Skattefar", "Taxdaddy" - and it's the name of their twitter account: https://twitter.com/Skattefar