What are diffence between runnig program which use jre 1.2 & running them on J2re 1.4?

Hey, Most of the program I have run on jre 1.2 but

will upgrading to j2re 1.4 make any difference to them?

and what Incompatibility can occur?

I need to run a program, which need j2re 1.4 or j2re 1.3

but I don't know what effect I will have on the rest

of my programs which use jre1.2