This is the answer I give to IT people when they ask if it's okay to customize the Windows boot logo.
DO NOT DO THIS ON A PRODUCTION MACHINE OR YOU WILL REGRET IT.
If you hack the bitmap everything will seem fine until six months down the road when you decide to install the latest service pack. The service pack installer will not upgrade your ntoskrnl because it looks at the file and says "Hm, this isn't the standard uniprocessor ntoskrnl, it's not the standard multiprocessor ntoskrnl, it's not the standard advanced server ntoskrnl, I can't tell what this is, so I don't know which version of ntoskrnl to update it to. I'll just leave it alone."
If you are lucky you will bluescreen at boot because the old ntoskrnl is incompatible with some other critical part of the service pack.
If you are unlucky, your machine will appear to run normally when in fact it is quietly corrupting itself, and then it will keel over or generate bogus data when you least expect it.
If you planned ahead, you will have quit your job and moved to Hawaii so the disaster falls on your replacement's head to clean up while you sit on the beach sipping a pina colada.