This is propebly not an answer, but still some info. After years with sme server I have learned that there is only one way to make a sme server unstable or less dependable and that is to modify it. No modifications will normally mean no problems.
I would guess upgrading to php5 is that kind of modification I would not do with a SME 7.3 installation.
What I actually do instead is to run SME server as a near unmodified virtual server. Then I also have a virtual Ubuntu and a virtual Centos server on the same PC box to do PHP5, if and when required. There has not been any issues or problems with this at all. Actually I am running "only" sme server most of the time. (Centos 64 + Vmware host system + virtual Smoothwall gateway + virtual sme 73, server only + virtual Windows 2000 Pro.)
There has not been a problem at all for the last six monts, and php5 can be done under virtual Ubuntu or virtual Centos, if/when required.