Java applets

i am working on a file uploader at my part time job, and i am having some problems getting it to load whithin a java 1.6 JRE on mac, windows, etc..... The source I am working from for the applet seems to target 1.4.2 and I am using those libraries for compilation. Several of the namespaces have been depricated since then so if I compile for 1.5 I get warnings, and I have not tried compiling for java 1.6.

Does anyone have any idea what rabbit holes I should chase on this?

It works fine in a java <=1.5 jre.

I am thinking that I need to add some JNLP code to load a specific JRE, if they have java 1.6 installed since it will perform the JNLP if it has 1.6, else it will ignore it, and I think that will work, but i wanted some hacker advice.