[CHINESE] Reference of the detail/ Source of this article: http://forum.51nb.com/viewthread.php?tid=1548345&extra=&highlight=&page=1
[CHINESE] Forum page to buy this modification suite: http://forum.51nb.com/viewthread.php?tid=1548947&extra=&highlight=&page=1
[CHINESE] FHD modification for T420/430 series are also available via a customed LVDS to eDP board: http://forum.51nb.com/thread-1552978-1-1.html In plain terms, the EDID information must be coded on the external LVDS to eDP board so that no BIOS modification are needed.
Firstly, I am NOT the inventor of this method, he who achieved this modification did not give details about his modification in some way. I am only a customer who bought his modified adapter.
I’m just trying to guess what he did to my motherboard and some of my descriptions below might be totally wrong and yes I will mark them specially.

 
Here are the main(brief) ideas of the following modification:

• Renounce the video signals produced by original LVDP lane because it cannot support a screen resolution higher than 1440×900
• Use the signal output on dock station to transfer data because it supports DisplayPort
• (I’m not quite sure about this part)’Hijack’ the signal of original LVDS lane (33 PANEL_BKLT_CTRL and 33 BACKLIGHT_ON ) input so as to control the backlight of the new screen panel

An Overview of LVDS/eDP Adapter

Modifications on Motherboard(Video Signals, w/o backlight control)

 Yes, there is an applicable solution to modify(replace) the built-in LCD screen to Full HD resolution (1920 * 1080). Yes, it still supports one other external screen either analogical(VGA) or digital(DisplayPort).

Firstly I must declare that I am not the inventor of this solution, and I personally hold no responsibility to any thing that might happen on your laptop.
kingkonglue in forum.51nb.com is the one who invented this solution, I take no credit for the solution introduced below.

Background Info

As almost everyone knows, the X220/X230 series uses a one-lane(single channel) LVDP lane to transmit video signal to the build-in screen panel, which only supports a screen resolution  lower than 1440*900. This is the main obstacle to install a FHD panel on the laptop.
There are at least 3 solutions available to deal with this problem.
Leokim is the first person to achieve this goal, he successfully modified his X220 with a Full HD panel (which seems to be a touchable screen toke apart from a Dell XPS 12” series, LP125WF1). His solution is not quite clearly explained, what we know till now is the modified BIOS and signal wire and the BIOS modification is hard for X230 series due to the encryption of UEFI BIOS.
The second trial of such modification is done by 东大师(Dong Da-shi). Presumably he used a self-made  LVDS–>EDP adapter to fit the screen panel , and added some signal wires to the original LVDP signal wire in order to support higher resolution. This solution has a chance to encounter signal interference because of the modified analogue signal wire(LVDS).
And I myself heard somehow there is another `shortcut` toward such modification through the interface of  dock. As we know there are two digital video output available through the external dock and hence we can make use of one of them. However, problem remains on the backlight control of the panel.

Screen Panel

In 2014 there are more high resolution panels available for the 12.5″ X series. After Dell XPS, ThinkPad X24o starts to use eDP video lane instead of LVDS for the sake of  FHD support. As is written above, filling the gap between LVDS and eDP is the key to our modification task and there exist at least two main problem on the way of achievement.
BTW the model of screen panel that I used is LP125WF2 SPB1 (I guess that all `LP125WF2` series are compatible).

FHD Signal

WIP

Backlight Control

WIP

Problems Remained

(Updated on Aug 29 2015)
Although we have successfully connected the FHD panel onto the mainboard, small problems still exists in everyday use. The following problems might occur:

• A Higher Temperature on GPU/CPU, especially when you attach a external screen.  This is because that the original output is still alive, actually the system will recognize TWO screen panels with the original one disabled partially. The same video signal will still be outputted to the original port. When you attach a 2nd screen (like a external monitor) the system will have to maintain the screen output of 3 panels actually.
• Bugs on the Driver –> 16-bit-like color depth (on X230 series; the X220 series are exempted). I was encountered with this trouble as many other people who holds the x230 laptop.  However, x220 series will not be affected if it uses a modified driver provided by the inventor of the adapter. This bug will cause a 16-bit-like color output on  the screen panel, such that the gradual change of one color will not be so smooth as it was. If anyone remember the 16-bit color depth on the earlier LCD panels, that’s the feel. The root of this bug lies in the display driver, but no solutions has been ever given till now.
• Black Screen on the First Attach of External  Screen. The earlier version of adapter (like what I owned) will encounter this bug, but it is reported solved by an upgrade to the chip on the adapter (on the hardware I mean).

WORK IN PROGRESS.
THIS ARTICLE WILL BE WRITTEN UP IF I HAVE ANY SPARE TIME.