Swap internal parts between two iMac 21.5, possible?

Yes you can put the 2010 GPU board into your 2011 system. Just keep in mind you won’t get the same performance if you got the better board in your 2011.

Here’s a good guide on the process iMac Intel 21,5" EMC 2389 GPU Karte austauschen

You’ll need both CPU/GPU thermal paste for the GPU and you’ll need thermal pads for the VRAM chips. Once you’ve clean off the old junk dry fit the heat sink and use a few strips of card stock to use to figure out the gap between the chips and the heat sink you want it just snug. Cut the blocks so they are just over the edge of the chip as over time the pad will shrink a bit.

Here’s a source for the pad material:

• 100mm x100mm x0.5mm Conductive Silicone Thermal Pad
• 100mm x100mm x1.0mm Conductive Silicone Thermal Pad
• 100mm x100mm x1.5mm Conductive Silicone Thermal Pad