If it's still working properly, then I don't see any reason to invest in a new one unless the old one isn't sized properly or is having reliability issues. The 3-4% AFUE jump isn't enough to warrant the replacement cost in my opinion.

Aside from the question of improved efficiency, which may be minimal, there is the concern about the expected life of the boiler. You may be approaching the limit. When they quit they usually do so at the most inconvenient time and, if they spring a leak, you'll have a big mess to clean up. Better to replace while you're ahead of the game.

If it's a steel tube boiler, you need to start thinking about a replacement.

If it's a cast iron tube boiler, you probably have many, many years left.

I have a Munchkin boiler. I believe the afue is @ 94%, so efficient it vents out of PVC pipe.

Thats great MSM

Have you started replacing control boards and venter assemblies yet?