This thing with "upgradeability" is highly theoretical I think.
I don't think I've ever actually done it, at least not motherboard/CPU wise.
Better to just buy the best price/performance ratio of the day, and the next time you want to upgrade in a year or two or three, do the same again, with whatever tech. has the best price/performance then.
Also, if you plan on running windows and not stealing it, you will need a new license again next time anyhow, and motherboard is usually a minor cost in the setup.
Only thing I've ever upgraded is graphics card and HD/RAM
My 2c.