What you need to do first is to lay out the requirements. Is this is using an installer, or will it be more of an xcopy type of install? If this is using an installer, you'll need to work out the steps required to do an unattended upgrade using the installer first before even thinking about PowerShell. You can invoke an uninstall of an application that appears in Add/Remove Programs without too much difficulty, but I've had mixed results with the process. e.g.
38# $app = Get-WmiObject -Query 'SELECT * FROM win32_product WHERE name = "VMware ThinApp"'
39# $app.Uninstall()
Shay
wrote a great article a while back on how to work with remote processes, check that out.