As good as it sound to convert it to Joomla 1.5 native mode, there are some disadvantages: Currently I am adding lots of new features to uddeIM and there are still lots of changes on my todo list, so when you start to convert uddeIM on your own, these two branches will be definitely out of sync. I have also some plans for a major redesign when all of my ideas have been implemented.
I have plans that uddeIM runs in native mode under J1.5 in future but currently it does not make any sense. The compatibility layer works fine and why shall I move all functions from the compatibility header into uddeIM? Just to say, hey its now running in native mode? Another problem is that there are still some bugs in Joomla 1.5 API so I wait until J1.5 has become grown-up.
The plan is that uddeIM will be compatible to J1.0 and J1.5 in future (there are still lots of J1.0 sites, think of all commercial sites which cannot upgrade immediately like a private site). And when you check the code you will find lots of functions which do already use native J1.5 functions when available.
My suggestion is to spend time on testing or to spend time on developing some new modules which will improve uddeIM. That would really help a lot.
E.g. I do still need an AJAX module for checking of new messages.