additional visit for different reason on UK standard visitor visa

Hi,
I was granted a Standard Visitor visa for me and my spouse for 1 week 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 PLAB 2 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 additional visit necessary to my visa center or will a clarification (along with supporting documents) on the next application suffice ? I will need to apply for a visa again once I pass for identity check purposes, hence I do not want to damage my future applications.
thanks and regards