VQOTW: FLAT RATE PRE-REGISTRATION VAT

My client is about to register for VAT and wants to use the flat rate scheme as she meets the conditions and it would be beneficial to her. However, her intention is to register for normal VAT initially, in order to recover pre-registration input tax, and then after the first return has been filed, switch to the flat rate scheme. Is this acceptable or will HMRC challenge it?