The reality is that the 02x,03x and 04x packages are compiled using lib5 and a 2.0.x kernel. So any binaries used in those packages will run on any of the systems. The only exception is possibly if a package uses the /proc/ directory or direct kernel access and tries to read some of the networking capabilities which were changed in the 2.0.40 kernel for the 04x series. The only real issue is the directory structure and the scripting. So what I do is to create a new package from scratch using the "Pack" packager for the 04x series. It provides all of the new scripting and directory structure needed by the 04x system. I then manually copy the necessary files into the new package structure. After that I look inside the install script to see if anything special is needed and make changes to the new script. This also applies to the rc_xxxx script to make sure that any special commands are included for stopping, starting and such. Sometimes the original package was not created in a normal manor and I need to make some directory changes or special links to make it all work correctly. But in most cases it is pretty straight forward and I can re-create a new package from an older package in a pretty short amount of time. On some packages I will go through the trouble of getting extra translations made if there is something more than what the default 'Pack' command creates. Which does take some time to send the wording to Dingetje for Dutch and Rene for Spanish (Any others are welcome).
Of course because I say quite easy for me does not necessarily mean REALLY easy and I am pretty sure that having designed and built the 'Pack' packaging script along with the 03x and 04x package structure and system gives me a little bit of extra knowledge about what can be done and how to do it. Because there are a LOT of new capabilities in the new package scripting and structure that never used to exist and in reality those new feature were the driving force behind moving from the 03x series to the 04x series. Because maintaining backwards compatibility was to restrictive.
If you are afraid that you might make a mistake. The chances are high that you will never learn anything.