for the short term there will be a big dump. but no long term impact. I don't think they're all stupid, they will sell gradually and announce all of it after they finish selling it. so no one realizes that they have sold their bitcoins. and when the announcement is published there will be a momentary dump.
Selling gradually would be possible until they will sold all of the Bitcoins they're holding.
With regards to the announcement, it might cause further dump after Tesla selling their holdings so in short it will be a double dump. There might be a short term dump after selling and their announcement will cause more dump and it will be a disaster for some investors but an opportunity for some.
If this will happen, it will only have an effect on the short term to mid term but this will not have any effect in the long term since many will buy Bitcoins at a lower price.