The date for the grant change is 1st March. However :
To encourage zero emission vehicles and maximise the number of everyday motorists who can benefit, a price cap for Category 2 and 3 vehicles will operate from 1 March. Vehicles that cost £60,000 and above will no longer be eligible for the grant. There will be a derogation for Category 1 vehicles, which will remain eligible for the full granth
So, if I read that correctly, Tesla is a Cat 1 car, so will still be eligible for the grant, but the grant will be £4,500 from 1st March.