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Switching from JVM 1.4 to 1.5 has performance benefits as per release notes.
http://java.sun.com/j2se/1.5.0/docs/relnotes/features.html#performance
We have Java 1.4 compiled classes which are run on 1.5 JVM, will these classes suffer in performance because they were compiled using 1.4 ?
Many of these features are “syntactic sugar”. Meaning the language has new features, but it compiles to the same bytecode. So no, you shouldn’t suffer in performance. Same bytecode on an updated VM.
http://java.sun.com/j2se/1.5.0/compatibility.html#incompatibilities
Also, depending on who coded your 1.4 classes, they may have had intimate knowledge of a particular VM/gc implementation and optimized for that. Not likely, but who knows.
The best way to know hos something performs is to run & profile. “Measure, don’t guess.”