disclaimer: i performed this mod several months ago and am writing this post by recollection alone, for the sake of documenting some stuff i had previously documented on the fascist shithole that is Twitter meaning that everything i wrote is now gone
so the standard configuration of a ThinkPad X61t is a 1024x768 (XGA) screen. there are some 1400x1050 (SXGA+) screens out in the wild for the X61t, and people have used them to retro-fit them into the X61 (non-wacom version of the X61t). i have owned a X61t since 2017 and have been trying to find the SXGA+ upgrade panel for a while since it's my main tablet for doing art and having more screen real estate means i can continue to use it in the face of mindless modern user interfaces that don't care for reasonable computer resolutions anymore. Krita is one such example where a 1024x768 screen is not sufficient for its unnecessarily humongous brush editor (which doesn't really offer that much different from, say, Photoshop 7.0's, which is more than content with a screen that size. go figure).
in 2022 i found an ebay seller selling this panel, together with a sketchy-looking LED backlight mod that seems to be a clone of an Unboosty Blue. i chose not to install this backlight, even though it sorely needs it, in hopes one day i can find a proper daylight mod. the panel, however, came to me already modified: it didn't have the acrylic sheet glued in front of it that provides a X61t's LCD protection from the pressure of the tablet stylus. it was at this point that i remembered all the scattered forum posts saying without much detail how to use a X61t panel in a X61t you need to "unglue the digitizer" and how that (from my experience with this one panel) isn't true at all.
for the record the specific part i bought is a HV121P01-101 (FRU 93P5609). this is known to be a X61t part, not a X61 one. and you can clearly see on the back of the panel a secondary flex board with a large "WACOM"-branded chip on it. this is the actual digitizer, and it sits on the back of the panel, behind the diffuser layers. the wacom pens work with a magnetic field (and for the most part wacom technology hasn't changed in decades, since i can use my X61t pen with a Samsung P585m, and vice-versa) which means that they don't sit on top of the screen. the "glass" that these forum posts mention having to unglue then, i posit, is actually the thick acrylic protective layer that my stock X61t panel has.
since the new SXGA+ panel was already missing it, i had to transfer the acrylic sheet and the strips that stick it to the edges of the panel onto the new one. the resulting sandwich is the same thickness as the original panel and fit my X61t perfectly. unfortunately if you're not careful and working on a very clean surface, you can get debris stuck between the acrylic and the panel, which i did, and now there's very fine hairs stuck on my screen. they're easy to ignore, but one day i want to reassemble it cleaner.
also, i was able to indeed verify (while inspecting whether it was worth it to try to instal the cheap LED mod) that the wacom layer is completely separate from the panel and sits with the backlight/diffuser layers. therefore i posit that the opposite of poaching X61t panels for X61 upgrades can be done: we should be able to poach X61 SXGA+ upgrades (or any other panel that matches the size and LVDS pinout of the X61t) and transplant the diffuser layers onto the bare LCD to make it wacom-enabled like the original.
in the end, i am now a happy owner of a X61t with a brand new battery and a 1400x1050 screen. i'm now able to make my art more comfortably and wherever i want. it's a good bit of kit and a really cheap way to get a screen tablet!