It is not the end of the world, but understand you may have some issues. By not replacing all at the same time, especially if they are not the same size or type, the charger will charge one bank over another more so or as needed, and will possibly neglect the other bank from time to time. It realy depends on your charger. always stick with like style batteries, flooded, gel, or AGM, never ever mix them, I would not mix them even on different banks since your charger will have various settings for each type and cannot charge be setup to charge 2 types. Some chargers can handle AGM and flooded off the same setting, but they are more ideal for Flooded then AGM. Personally I would replace all of them at the same time, but if your budget does not allow that, replace a bank at a time and try to find the same batteries if possible and they suit your boating needs, or a very similar sized battery so they handle the loads similarly, although the older bank may need to charge longer or harder if they are showing any wear or neglect. Batteries aren;t cheap, I'm replacing mine in about 2 weeks, between Optima's and Dihard Platinum's, my boat only fits Group 24 or 34 size batteries, there isn;t room for 27's so I am limited. To have more battery capacity, AGM's or Gel's are usually the way to go, there are some bigger flooded batteries in that size range but they often have more issues and last shorter from my experience. The Optima's and Diehard's are $200 each, I will be replacing all 3 in about 2 weeks, just trying to decide which to go with. Leaning toward's the Diehard's but I would need a new charger since mine won't handle AGM's, but it is a junk Guest charger anyways and needs replacement so to me that is not a factor.
