If you are Accredited (base tier), you can switch to Accredited+ at any time of the year provided you have all the pre-requisites. Switching to Accreditation+ means that you will be paying the fee in full for 12 months of Accreditation+.
If you are an Accredited+ coach, you will need to wait until your next renewal window before you can switch to Accreditation (base tier). Your renewal window is the one-month period before your Accreditation expires.