We've had a similar issue with our 205s beyond the stated memory deficiency.
What we did was re-allocate the older 205s so that they would be together and so we wouldn't have to worry about that "mixed" cluster issue. I would say you don't have to abandon them, but if you MUST use them in a cluster with the newer 205s, you may be out of luck.
In a pinch you could also run two clusters (if your environment is suitable to such a thing)