Do you have any PrE 10 Projects, that are not complete?
If you do, then you will probably have the best luck by finishing those in PrE 10, before Uninstalling it from the computer.
I agree with Bill. Keep version 10 until you've finished all of your version 10 projects. Then start your new projects in version 12.
Definitely do NOT open a version 10 project in version 12.
In view of more recent developments as highlighted in another thread, does the focus of this thread now become non relevant?
1. Premiere Elements 10 as well as 12 is supported by Windows 8.1 64 bit. Premiere Elements 10 will have problems on this operating system because of the NVIDIA video card issue specific to version 10. Premiere Elements 12 will have no problems on this operating system because it is unaffected by the NVIDIA video card issue specific to version 10.
2. You have now found a suitable older NVIDIA driver version which is allowing you to continue with successful use of Premiere Elements 10 as your video editor on your Windows 8.1 64 bit computer.
If you had decided to keep 10 and 12, just use one at a time. Download Content for 12 from within the opened project at the level of Tools/Movie Menu/. Please see blog posts.