I'm plotting my first native NVMe build, and intend to reuse the existing Windows 10 install on a Samsung 850 EVO SSD. I haven't done this before, and falling back on "just reinstall from cold" is ...

If you're cloning the original SSD to the new SSD, an external enclosure is needed. This option from ElecGear fits M.2 PCIe NVMe SSDs and can connect back to your ThinkPad T490s with USB-C or USB-A.

If you only use your desktop PC to play games, browse some websites, watch some videos, and occasionally transfer files here and there, you won't notice much of a benefit from NVMe SSD caching.