you wish to increase your borrowing term by 5 years? and your fixed term upped by 3 years. It may be worth asking the lender "just as a general enquiry" and seeing what the difference in terms of costs average out to. It is dependant on the lender whether they would process a new application or just alter your existing one.
Its always worth a general enquiry to the lender

Folks,

We've had our mortgage accepted and are well underway with buying the property we want. Currently our application details are Halifax 2 years fixed @ 6.2 over 25 yrs, we would however like to change those details to 5 yrs fixed and 30 years. Would changing these details now affect our eligibility for the application and would another credit score need to be done?

Thanks in advance