In my opinion, I don't think I'd try switching them from the dead printer (if sitting in it for about 6 months and not being touched) into the 6400. Chance there may be something that happened to the ink formula in that time frame while in the printer (but I could be wrong). Now, if it was about a month or sooner, probably have no issues since both models use the same inks. Probably check with Canon to verify this.
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