msiexec /update PSE450w2k3x64R06.msp CTX_USE_EXISTING_JRE=YES
The CTX_USE_EXISTING_JRE=Yes will ignore the install stopping Java check that forces you to use a certain version (that seems to always change).
The only thing to note is that on 4.5/5.0 you will likely lose the ability to use the XenApp Advanced Configuration tool (formally the CMC) on this server, so you'll want to make sure you have at least one server that is updated to the latest Java required for the latest HFR patch (like your dedicated Data Collector I know we all have right?) - otherwise you won't have access to the handful of items still in the older Java based CMC tool.
Sometimes disabling the Java check is needed, and by adding this switch you can remove the Java requirement for any Citrix patch, hotfix, hotfix rollup or even the Citrix install itself with only a small loss of functionality to the CMC tool