I have one of the "old IZT's (331-2400 series) which is failing. About 2 years ago it was noisy and running warm, and I changed the filters and fluid in both. The failing one apparenlty had gotten some water in the fluid and had some rust on some of the parts. After replacing the filter fluid it was better- but still louder and sl warmer running. A few weeks ago - after being unused much this spring due to repeated rain- it was much noisier and had little power on that side. I have taken it off- opened it up, and found the fluid has small metal "dust" in it. I have cleaned it out, have reassembled and am running it with a drill. It still does not sound good- but is better. I am tempted to put it back on and run it until it is finally dead and then replace. Will the new 2800 series simply bolt on in place of the old ones? Any problems with replacing them one at a time? Cost?