will I have to pay more?
No you won't ! The medical examination (s) are EXACTLY the same (with extras such as ECG or audio, if required), for both renewals and revalidations !
This is PURE UNADULTERATED BUREAUCRACY ! ! Typical CAA and JAA ! !
The dating arrangements work almost identically to the car MoT, ie you can renew in advance of expiry (within 45 days prior) without losing any validity.
A REVALIDATION is if you have your medical during the 45 days prior to the expiry of your last certificate.
A RENEWAL is if you allow the previous certificate to expire . . . . then later have a further medical for another certificate.
Easy, innit ? !