You will need to contact your current training region and training programme director as soon as possible after you have accepted an offer. This will have the additional benefit of allowing your training programme director to identify a gap in the programme ahead of HST recruitment, giving them the opportunity to find a new trainee.
Your stroke medicine training will be classed as 'out of programme (training)' and your training region is responsible for approving this; which is why it is strongly recommended to discuss it with them before you apply to stroke medicine.
The JRCPTB website has guidance on out of programme, including the form you will need to submit to them, so that they can record your new programme and add stroke medicine to your ePortfolio.