Once you have the ok,they will allow you to do this right away, ring up a dealer if you know what you want,tell them you can exchange asap.
If they have a stock car you will be able to go ahead and not wait, be aware if you need to see the cars on the 12th April,make an appointment as soon as you can ,as social distancing will only allow bookings to go in showrooms and after the long time closed could be busy.