Upvote:4
IMHO this is a significant change, as the only reason for PLAB2 is to practise medicine in the UK, signifying your intent to work and live there. Surely, you planned to sit the exam prior to your recent visit (and visa application), as you must have already taken and passed PLAB1.
However, that you were issued a visitor visa is in your favor. Rather than risk an entry refusal at the border, or a refusal of a subsequent visa application, it may be advisable to notify UKVI of your plans.