Cost of moving from micro-entrepreneur to auto-entrepreneur?

After reading alot of information about changing from micro-entrepreneur to a regime micro-social simplifie, I thought that this would be better for us financially due to our low joint earnings. Since 2008 we have been registered as a micro-entrepreneur business in France with my wife as conjoint collaborateur. I understood that as we have low earnings, approximately £10,000 per annum, that it would be better to pay our social charges/cotisations only when we earn, then being approximately 23% as a carpenter/artisan.
 
I asked RSI to change us and they have confirmed we will change to an auto-entrepreneur with effect from 1 Jan 2013. But we have now also received an Appel de Cotisations asking for 340 euros per month (3500 euros over 2013). We have been to the centre de paiement du RSI and they have said that we must pay this even though we are changing to this new system. Is this correct?
 
I don’t understand, I thought that this was the benefit of changing to pay and an exact percentage at the end of each month or 3 months. If it is, won’t we then be paying more every year being the amount on the appel de cotisations plus the percentage (23%) of our earnings when we declare our income? Can you please help as it is causing us financial problems and they are taking the first amount out from our bank account on 7 Jan 2013.