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.
I strongly advise you the Epson ink, I used Inktec ink before in an Epson XP-422 and since almost a year now I use an Epson ET 7700 with inkttanks.
The Epson ink nrs 105 pigment Black and the 106 photo Black, cyan, yellow and magenta is much much better than the Inktec inks for fading.
I did put some test photos a year ago of the Inktec and the Epson 105/106 ink in the Windows with in the morning direct sunlight and the Epson photos stay mu ch better.
And the photo prints of the ET 7700 are very good.
There is also the Epson ET 7750 which can print A3 and uses the same ink.
I don't know if you can fill the tanks with Epson ink when there should be a tiny bit of the Inktec still in the tanks and in the printhead, maybe you ask the Epson service point for advice.
I wish you all the best.