I have Installed 3.5.1 over 3.5 using the "Update" option on my Mac Mini 1.5 with no issues. I have also updated 3.5.1 over 3.4 on my PowerBook 1.67 without any issues (I had to backup my 1.23 version of OWB first though).
Thanks to the OS Developers!
[ Edited by IKE On 2014/2/17 11:12 ]
MacMini G4 1.5Ghz/PowerBook G4 1.67Ghz/PowerMac G5 2.0Ghz DP 7,2 Radeon 9650/256MB