I got a Standard Visitor visa for me and my spouse for tourism purposes in November 2018 (valid until May 2019). We did visit and return precisely on the dates stated on the application. Now I have a PLAB2 examination scheduled in March 2019 (not disclosed in the application previously). As my visa is still valid, does this count as a "change in reason of visit" and is disclosure of this necessary to my visa centre? I will need to apply for a visa again once I pass for identity check purposes, hence I do not want to damage any future applications.
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.