Moving this discussion to the Photoshop Elements forum. It is not required for you to remove Photoshop Elements 8 prior to installing Photoshop Elements 10.
Both programs are self contained so PSE8 and PSE10 will work happily side by side.
I have versions 6, 8, 9 & 10 on my machine.
Any custom actions, plug-ins etc would have to be moved manually although you could carry on using them in PSE8.