What components can I change in my system before I invalidate the OEM License?
There is a general consensus that the OEM license is paired with the motherboard. You can generally change any other component bar the case where the COA is attached. If you change the case, and the COA isn't physically attached, this breaks the terms of the EULA. Also, if you do buy a new OEM license, but don't attach the COA, the same applies, you are not licensed. If you change a significant amount of hardware you may be asked to reactivate, but this doesn’t mean that you are necessarily unlicensed.
However, you can change the motherboard under a warranty claim if the motherboard was faulty. A warrenty claim is defined as replacement of the original board from the company that your purchased it from, under a warrenty scheme. You cannot simply buy yourself another board if there was a problem with your existing motherboard and still be fully licensed. If you are simply upgrading for feature or performance enhancements, you would need a new OS license.