The complexity of the migrations depends on the amount of vendor libs/classes you are using in your application.
If you are using only Java EE compliant features then it should not be an issue. Some notes:
1) JBoss 6 has not yet been published as stable release. At the moment with the M5 you can get most of Java EE 1.6 features but you have to wait the end of the year (I guess) for the stable release. However, since you are not using any Java EE 1.6 feature in your project, I'd advice using JBoss 5.1.0.
2) You have to upgrade to JDK 1.6, because with JDK 1.4 you cannot even start JBoss 5/6 and JDK 1.5 has reached its End of Service Life (EOSL) on November 3 2009. This means you will not get support/patches from Oracle, unless you sign a support contract with Oracle.
3) Time ago I started to write a guide for migrating from Weblogic to JBoss. Have a look if it can helps.
Thank you Francesco.
How can I justify business that JBOSS 5.1.0 is good to go with Production and not with latest. Because everyone wants latest.
From my end for production system, I always prefer stable product which is there for a while and being rectified with patches.
Let me know your thoughts.
Well from my point of view developers are mostly interested to get hands on the latest stuff but end users and the customer-who pays for the software- has a wisely and conservative approach like you. I think you should point out two things: at first you are not using any Java EE 1.6 feature so there's no hurry to move to JBoss 6. If that's not enough, you can still show to your staff the bug list of the very first JBoss 5.0.0 release (luckily the release 5.1.0 is much much better :-) )
Hope it helps,