EJB3 applications developed on 4.x will require some minimal modifications for deployment on 5.0, which is targeted for release late spring. The modifications will be concerning the jboss extensions to the spec, such as package names of jboss extension annotations. There will also likely be configuration changes required (e.g. ejb3-interceptors-aop.xml).
In a nutshell, the spec based implementations should port fine - it's the extensions that could require some work.
From an EJB3 perspective, AS 4.2.2.GA and AS 5.0.0.Beta4 are closely-aligned. When making the jump to the 5-series, most of my previous applications' migration issues centered around packaging and declaration of dependencies (as the underlying container, MC, has changed, along with many core services and ObjectNames).
If I were them, I'd try deploying their existing EARs/JARs on each platform and see where the differences lie. Given that they've got some time before their deliverables, I'd try to get on the AS5 train early as possible. 4.2.2 is certainly more stable, but they want to be developing alongside our own releases to stay in line incrementally instead of worrying about a large migration later.
Also good news for this type of scenario is that we'll soon be allowing existing AS installations to upgrade EJB3 without the entirety of the AS (via our new/reinstated Plugin Installer).
Where are you giving the training?
Also good news for this type of scenario is that we'll soon be allowing existing AS installations to upgrade EJB3 without the entirety of the AS
This should read: "...existing AS 5.x installations ...".