There's no generic reason why a 1990 installation should need any work doing on it in order to swap the CU.
BUT a lot can have happened to it in 25 years, so your specific installation might need things done - circuits/cables might be damaged, additions using the wrong cable or topology may have been added and so on.
Two things to consider:
A significant introduction into yours will be RCD protection, and that can uncover faults, or faulty appliances, of which you were unaware.
You might want to take the opportunity to split circuits up and/or add new ones, for example if you have one lighting circuit, changing it to two - upstairs and downstairs, and that might mean a bit of modifying how the landing light is wired.
Do not go with anybody who plans to just replace the CU and fix any problems encountered as he goes - what you want is the existing installation checked out first so that you're aware of any problems which need fixing beforehand.
An all-RCBO CU is better than one with a couple of RCDs, and probably not a lot more expensive.
As ever, personal recommendations are always the best way to find a reputable tradesman, but if you're having to go ahead without much in the way of those, or references, don't put any store by registration itself - sadly it is possible to become registered with woefully inadequate qualifications and zero practical experience. You don't have to spend long here to see people cropping up who are registered and "qualified", but who are clearly seriously incompetent in reality and who should not be charging for their services.
It's your money, £'00s of it, and you have every right to ask prospective tradesmen what their qualifications are.
Just being listed here is not a good enough guide. No genuinely experienced electrician, with the "full set" of C&G qualifications will mind you asking - in fact he will wish that everyone was like you.
I feel sorry for people who have been misled by training organisations and (shamefully) the Competent Person scheme organisers into thinking that a 5-day training course, a couple of trivial examples of their work and some basic understanding of how to use test equipment will make them an electrician, but not sorry enough to agree with them trying to sell their services to Joe Public.