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T450 has Wireless adapter from Inter whereas T420 has Sierra according to Lenovo Support Website:<br />
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If you want to swap the wireless cards, you would need to install the correct driver.<br />
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Intel Wireless Adapter in T450 is compatible up to WIndows 10 64 bit, whereas Sierra is compatible up to Windows 7 64 bit according to Lenovo Support.<br />
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So if you are going to swap the Wireless adapters, you'd need to deal with the driver and also depends on the operating system.<br />
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Check below:<br />
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<a href="http://support.lenovo.com/us/en/products/Laptops-and-netbooks/ThinkPad-T-Series-laptops/ThinkPad-T420?tabName=Downloads&linkTrack=Mast:SubNav:Support:Drivers" rel="ugc">http://support.lenovo.com/us/en/products/Laptops-and-netbooks/ThinkPad-T-Series-laptops/ThinkPad-T420?tabName=Downloads&linkTrack=Mast:SubNav:Support:Drivers</a>&nbsp;and Software|Drivers and Software&amp;beta=false<br />
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<a href="http://support.lenovo.com/us/en/products/Laptops-and-netbooks/ThinkPad-T-Series-laptops/ThinkPad-T450?tabName=Downloads&linkTrack=Mast:SubNav:Support:Drivers" rel="ugc">http://support.lenovo.com/us/en/products/Laptops-and-netbooks/ThinkPad-T-Series-laptops/ThinkPad-T450?tabName=Downloads&linkTrack=Mast:SubNav:Support:Drivers</a>&nbsp;and Software|Drivers and Software&amp;beta=false
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Sierra is a cellular card, not the Wi-Fi card. That would be a different card.
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I want to swap them out <br />
<br />
I dont have the t420s in front of me, Ive to go onsite and wondering if I can swap my own card with the customers ...
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I want to swap them out 

I dont have the t420s in front of me, Ive to go onsite and wondering if I can swap my own card with the customers ...

Lenovo t420s and t520 wireless cards - are they compatible?

A laptop or notebook is a portable personal computer with a clamshell form factor, suitable for mobile use. Although originally there was a distinction between laptops and notebooks, the former being bigger and heavier than the latter, there is often no longer any difference. Laptops are commonly used in a variety of settings, such as at work, in education, and for personal multimedia. A laptop combines the components, inputs, outputs and capabilities of a desktop computer, including the display screen, speakers, a keyboard, and pointing devices (such as a touchpad or trackpad) into a single unit. The device can be powered either from a rechargeable battery or by mains electricity from an AC adapter.