That's right they back date it from the moment you cross the threshold, alot don't realise the threshold amount is the sum of a rolling 12 month period, your tab doesn't start again in April so to speak.
I remember when I went vat we had to back date approx 3 months, I stood to owe 9.5k from the off, I was advised to go back and re invoice the builders for the vat, which they paid no problem, obviously I had to pay vat on domestics done in that period out my own pocket.
But you can also claim back the vat you have paid on big purchases from the last 3 years, which meant after all tallied up I went into it with a cheque from the vat man for £8k. Not bad at all.