Gas boiler upgrade - 25kW or 32kW

I concur with what @antoinolachtnai says and I'll add that the reality these days is that many houses have also upgraded insulation and windows as well as getting walls pumped since the original install, so the demand on the boiler (through heatloss) can be fair bit lower. So if you house has had extensive upgrades such as those mentioned then the heatloss is now lower and therefore you may be able to operate off a smaller boiler thank was originally installed. Also don't forget that when choosing the boiler the original installer was working of a heatloss value and they then picked the next largest boiler, so there may have been some heatroom in that boiler's capacity in the first place.

Anyway, the right answer here is to perform a heatloss calculation and work off that value for the boiler.