php4 to php5

I am being pushed to update our prodction server from php 4.4.2 to php 5.2.5 because of an alert from ScanAlert claiming that there are vulnerabilities in any php version prior to 5.2.5. My concern is that we have several quite old php apps running on the machine and I hav neither the time nor the inclination to learn them well enough to debug and update them if they break. My question is what kind of problems am I likely to experience with the update? Thanks, --Richard