Fred, thx for your opinion.
For what's it's worth, I have this morning done the swap, which is in fairness quite straightforward and everything fell in place nicely; one just needs to be careful with the bezel so as not to break the clips - but the x220 is a marked improvement over previous models when it comes to disassembly and reassembly.
As regards my yellowish tinge, I have played a lot with the colour enhancements page in Intel HD settings and I have been able to get something closer to my liking, so my whites look whiter and not creamy but I just wished that a 300-NIT screen would have a colder colour temperature and be a bit brighter - although I know IPS is not as efficient as TN in terms of light transmission.
I can therefore now confirm that the IPS/TN swap is a simple conversion
. The x220i is on the other hand happily running fine with its TN panel too