ETASSUM45260 | Schedule 4 Company Share Option Plan (CSOP): Exchange of share options: CGT consequences for the option holder
From HM Revenue & Customs · Employee Tax Advantaged Share Scheme User Manual
For Capital Gains tax purposes a paragraph 27 exchange is not treated as involving a disposal of the old options by the option-holder (Section 237A TCGA 1992). The new rights are treated as being the same asset as the old options.