Well yes, in the case of a flat I recently assessed!
It’s was a pretty good flat, build in the 60/70s, had new double glazed UPVC windows and lovely 250mm thick loft insulation!
But it had a really old back boiler and massive cylinder tank!
The EPC rating came out at D 65, but with a simple upgrade to the boiler, it could have jumped to C 73 (C 74 by adding in TRVs too).
Putting in an A-rated boiler in the kitchen, such as the Worcester Bosch Greenstar 4000 25kW, would have given them more space in the flat, improved their energy efficiency and helped reduced their monthly gas bill!