That the inks are not chemically compatible. You'd need to flush the old ink. I don't really know how, probably by using a cleaning fluid.
I'd assume that if the printer was originally filled with T673 ink, and at some point it was mixed with the current ink and no harm occurred, the inverse process might go fine just as well. Don't take my word for that as I've not done it.
I see those Inktec liter bottles all the time at the stands in the local computer markets and before buying my L1800 they used to make me drool. If I hadn't read the question you posted I wouldn't have been confident that it would work. Heh, I'm sticking to Epson ink, but it's good to know.
I think the buying price is a rip off and the three month warranty where I live is ridiculous, but it seems to be a great printer so far.