Generally for a laptop the only modular parts are the battery, RAM, and hard drive. RAM is starting to be irreplaceable as well on ultrabooks.
You could replace the entire logic board (mobo + gpu) but it's probably going to be hella expensive. I know for Apple at least it's like 70% of the cost of a new laptop.
If it's soldered onto the mobo, you'll need to solder your replacement GPU on there as well probably?
I couldn't find an ifixit page for the laptop.
Here's lenovo's official hardware thing for your laptop
https://download.lenovo.com/consume...y400y500_hmm_1st_edition_sep_2012_english.pdf
1100 System board
For access, remove these FRUs in order:
• see “1010 Battery pack” on page 35
• see “1020 Dummy card” on page 36
• see “1030 Optical drive” on page 37
• see “1040 Hard disk drive(HDD)/Memory/Mini PCI Express Card slot
compartment cover” on page 38
• see “1050 Hard disk drive” on page 39
• see “1060 DIMM” on page 41
• see “1070 PCI Express Mini Card for wireless LAN/WAN” on page 42
• see “1080 Keyboard” on page 44
• see “1090 Keyboard bezel” on page 47
I don't see anything at all about the GPU, so I'm assuming it's attached to the 'system board'.
In the part list, it's item #40, "QIQY(5/6) VGA board". Nothing else is mentioned about it.
Good thing you still have Intel integrated graphics, right?