So much depends. Are they showing signs of failure? I certainly wouldn't just get new ones because of their age. Good agms that aren't overcharged can go a long time. With the extra cost involved in agms it is worth taking them as far as you can. If you are headed someplace where batteries aren't available (or agms in particular if you feel you have a need for them) then it might be worth replacing just for the heck of it but there certainly aren't any garentees there iether. Well, there might be but they won't do you any good if you are in the place where the batteries aren't and if you're in a place where they are then might as well replace them when they are no longer acceptable. Years don't nescessarily equate to cycles directly and so long as they have been left in fully charged state some agms are reported to have great storage
life, unlike wet cells which have high self discharge rates and require regular care. If they are in good shape and six years old the charging
set up is probably right and you may be fine for many more to come. You can check their condition realizing their specs and that degradation happens with time and use. Will they take full charge? Does amp hour drain equate to charge level? Ultimately, are they suitible for your intended use? Only you can answer. But I certainly wouldn't go looking to drop that kind of money
unless there was a reason. If you do, however, I'm sure there would be a happy cruiser in your neck of the woods who would take the old ones off your hands...