If you have made any changes to the JBoss AS configuration files, you will have to track down the exact changes you made. You did place the config files under version control so that you could track this, right? If not, download the exact same version of JBoss AS you are using and use a diff utility to tell you what you changed. For each change that you made, you need to determine how to make a similar change in 5.1.0. This usually means reading up about how 5.1.0 is configured. The biggest issue you will run into here is that many of the services that were configured using MBeans are now configured using POJOs in the microcontainer.
Any *-ds.xml files you have will probably work as-is in 5.1.0. There were no significant changes to *-ds.xml files. Unless you have declared various MBeans within them, in which case see the prior paragraph.
If you use messaging, you need to change how your destinations are declared. Look at the examples in docs/examples/jms/example-destinations-service.xml.
If you have any web services, you will need to regenerate the stubs and recompile the web service server and client code.
For your app itself, recompile it with the JARs from 5.1.0 and then deploy it and see what errors you get and then track them down and fix them.
If you have standalone apps, they will have to be recompiled with the JARs in the 5.1.0 client directory.
I probably forgot something, but for most apps this should be reasonably complete.