IO540 is right - you need to look into it very carefully to avoid inadvertently ending up with a very expensive aeroplane.
To give you a ball park idea, I was quoted £5k as a minimum, with a likely £7k as the final bill (including VAT) for a seven year old aircraft which wouldn't need any physical changes other than the registration re-painting.
I was advised to keep it on the N-reg and haven't regretted the decision yet.