I assume that "JBoss EAS" is a typo and you meant "JBoss EAP."
EAP 4.3 is based on AS 4.2.1 with JBoss Messaging replacing JBossMQ. In addtion, it includes various pacthes. Thus if you are running AS 4.2.x and either you do not use messaging or you have manually installed JBoss Messaging, then you should not have any difficulty moving to EAP.
If you are using JBossMQ, you might want to use EAP 4.2.
First of all, thanks for your reply, and second, I admit my fault with JBoss EAS typo, I thought it was called the same way as free JBoss AS + "Enterprise" ;-).
Therefore, talking about JBoss EAP, we're not currently using any messaging or any other "special" service from AS, then I will assume that an application currently running under JBoss AS 4.2.3 should properly work under JBoss EAP 4.2.
Thanks very much.