I want to upgrade my motherboard at some point down the line but I want to know if i have to buy windows again.

I'm currently using an OEM version of Windows 7 and some sources say i have to buy it again but others say i won't have do by doing some complicated things.

Also, I have been thinking about upgrading to Windows 10. If i replaced the motherboard would I be able to install Windows 10 instantly?  

I'm pretty sure that with Windows 7 if there are any significant hardware changes it just asks your for your key again. Not sure though.

I've also heard that if there are any hardware changes on Windows 10 you will have to purchase a key, so I'd recommend upgrading before swapping out your motherboard.  

I was thinking about installing Win10 on a USB, replace the motherboard and then install  

If you have your own copy of Windows then no, but if it came with your computer and you don't have the CD key then yes.