have had mine for three years now and keep thinking its time to replace them. but when i tune em up they still sound as good as they did when they were new.
I'm not going to try and tell you what you're hearing, but if you play your drums even on a semi-regular basis, then the only reason you don't
notice a difference is because the changes are minor and take place over a long period of time. I felt the way you do now once upon a time too, and when I finally did change my reso heads, I was surprised by the difference and improvement in the sound of my drums.
If you still like the way your drums sound, then it isn't a true necessity to replace them. Basically, it's up to you to decide if you want to drop a few bucks or not, but I say do it. Three years of service is pretty good for reso heads and they certainly don't owe you anything.
Keep in mind that swapping the old ones out for newer ones doesn't mean you have to throw the old ones away. In the event you don't hear a difference (I'm sure you will, but just in case) then keep them as back-up reso heads or put the old ones back on and keep the new ones for when you do need a change. Heck, you could even return the new ones to get your money back, or trade for some sticks or something if the store doesn't do refunds.
Nothing lost man.
As for my policy on when I change my reso heads, I usually do so every 2nd or 3rd time I change the batter heads, depending on the total time and amount of play they've seen...and of course the sound!