W
Welder
As for the PCB, the components being changed are incredibly simple. They go from one resistance to another and that sets the temp the boiler aims to hit. The components have a certain spec, and as long as the new components meet or exceed that spec I can't see how it could possibly affect the safety aspects of the boiler?
As you say, very simple. To be fair though if you were to replace you'd need to use pots designed for constant use. Those manus use will not be.
HOWEVER (the backside covering bit), the proposed mod MAY somehow contravene gas regulations.
What I'd do personally is one thing, but in your shoes I'd be contacting Gas Safe to ask for chapter and verse on this question as no one I know of here is qualified to answer this very specific question.
It does occur to me that if, for example, you are an electronics engineer and can demonstrate competence to undertake this task then your position may be different to that of us everyday Joes.
As advisors to the general public, everyone on here has a duty of care to give what one might term 'official lines' of advice.