I have the old Epson L850 (on its last legs, served me well) as I have 1 of each colours left over can I use them on the Epson 18100 as the colours are the same.
What's wrong with the L850 ? Do you rather like a larger A3 printer ?
But back to your question - yes - you may use the L850 ink in another printer, but please be aware that there might be (small) color differences which you would have to correct via the color adjustments in the driver or with dedicated icc-profiles, and it could be that the inks differ in the longevity of the inks , I didn't test either the T673 nor the 107 for their fading performance. You can unscrew the heads of the bottles with some force and use them on the bottles for the other printer.
Yes A3 was a priority also 18100 was the pick for quality glossy photos for the amateur, just lucky that colours are similar. Thank you so much for you help it saved €120 on inks, to say I am over the moon is an understatement.