Some of you might remember the T540p I got for free from work a couple of years back - after its mainboard died I rebuilt my older personal T540p with the the remaining good components and a donor W541 I bought on eBay.
I'm quite pleased with the result. Its revised specs are: i7 4930MX, Quadro K2100M, 32GB DDR3L RAM, AUO B156HAN01.2, an HP Blu-Ray drive and an Intel AX210 WIFI card.
And a few stickers obviously.

I did that too with my thinkpad, which is now my mediaserver. The mobo switch was surprisingly easy but the fan didn't fit so I had to order a fitting one which was a bit confusing. Overall it was pretty cheap and not that difficult though
I also needed to get a different Charger for the higher wattage and I had to change the serial numbers in the bios so it was able to indentify the other components, this was only needed to remove a warning on every boot