The following has also been pointed out to me:

Supplies prior to 1 April 2018
Where goods or services are supplied before 1 April 2018, the rate of 14% will apply to these supplies, irrespective of the fact that an invoice for such supply may only be issued on or after 1 April 2018, and the payment is received after that date.

Supplies commencing before and ending on or after 1 April 2018
Periodic or progressive supplies that commence prior to the effective date and end after the effective date will need to be split between the new and old rates. This split should be on a 'fair and reasonable' basis.

Supplies made on or after 1 April 2018
Supplies after 1 April 2018 will be subject to VAT at 15%. Please note that there are specific rules prohibiting vendors from issuing invoices or making payments prematurely in order to benefit from a lower Vat rate.