Is upgrading with the legacy driver in place the issue?
I read this article with particular interest as I have the same model of Thinkpad as Liam and as a Windows user, but with some Linux experience, have one eye on the October deadline.
Given the comment about trying ahead of time, I decided to adopt the "Live CD" approach to see how the laptop would behave depending on the video chip(s) were configured in FW. I therefore downloaded the latest versions of Ubuntu, Mint, Fedora, Opensuse and Debian. To my surprise, given Liam's experience, all of them worked quite happily with the Nvidia chip in Optimus mode.
I'm afraid I'm not very experienced in querying systemd based OS's for driver versions but viewing the dmesg files in Ubuntu and Mint, both of these were using nouveau version 1.4.0. For the record, Mint was based on Ubuntu 13.3.0 using kernel 6.8.0-51 whereas the Ubuntu Live image is Ubutu 14.2.0 using kernel 6.11.0-8. The other distro's reported using the Nvidia chip in the output of lspci.
I think the lesson from this is that the "average user" could quite easily migrate from Windows to any of these Linux distro's with only the learning curve of a new OS to face rather than the difficulties of mixing and matching kernel versions with proprietary drivers.
NB Obviously a quick test of each distro didn't stress the setup too far but if all the "average user" does is surf the web, use email, watch some videos and edit Office docs, I think they could quite happily use older HW rather than follow the MS landfill mandate.
Lastly, isn't it strange how we stick with outdated terms like Live CD when it's more likely to be a USB key and talk about taping TV programs when we're actually recording them to an HDD!