FWIW, I have now tested UT on a 5 year old laptop (ACER ASPIRE 5551) that has been upgraded to Windows 10 x64. I copied my UT folder over and ran the x64 executable - it worked without any problems given the following:
- ReduceMouseLag was set to false
- A valid CD/DVD key was set in the registry (for x64)
- In UT2004.ini, [Engine.Engine]RenderDevice=D3D9Drv.D3D9RenderDevice (for x64)
It even informed me that it had automatically turned on the Compatibility property on the executable for "Disable display scaling", because I use 125% DPI on the desktop. I always had to do that manually in previous versions of Windows.
So, another 10 years of UT2004 to look forward to.