How do I go about crediting an old invoice after 1st April 2018?

When doing a credit note for an old invoice, if you do not edit the line at all, just post the whole document as is, then it will keep the 14% VAT rate. As soon as you edit a line though, the VAT rate will refresh to 15%. So, to be on the safe side, you may prefer to do all old credit notes using your new 14% VAT code created for this purpose.