Depends entirely on which configuration (CLDC vs CDC) you're going to use, and what functionality and APIs your code needs.
If you're using a higher-end PDA, you can probably use a CDC stack and implement your application with minimal conversion, if I am understanding how the subset of Java in CDC works.
If you're going for something that'll run on a broader list of devices (targeting CLDC and MIDP), you're going to likely have to do some significant rewriting to fit into the APIs and the subset of the language that is supported on that level.
Either way, you'll need to load onto the device an environment for running Java apps. IBM has one (j9, otherwise known as the Websphere MicroEdition), and there are others out there that you could use.
Hope that helps.
Theodore Jonathan Casser
SCJP/SCSNI/SCBCD/SCWCD/SCDJWS/SCMAD/SCEA/MCTS/MCPD... and so many more letters than you can shake a stick at!