We have been in contact about this via e-mail and I extend it here in order to provide others with this information as well.
I have converted our old 9900 to Piezo Pro inks today. The printer was printing flawlessly for the last 8 years. I was aware from earlier conversions that yellow can be tricky for removing its cast. I was doing an init fill and then a KK3 and it looks like all inks - except for the ones in the Yellow and Orange channel look fine.
Yellow should be fine now after around 1 square meter of printing purge sheets through calibration mode. Orange seems to be tougher though. Do you have similar experiences? The problem with Orange is that it has the Warm Very light in it. I have been flushing and printing like crazy and have the feeling that it does not change the color anymore - however it looks like there is still some Orange cast. When can I know that it is where it should be?