What are the difficulties from moving from an entirely modal application to make it non-modal?

I have two applications in mind. Both are rather small, at least in the number of forms involved (about 40 each). They are similar in that both have hardly any input at all, as most of the data comes from a SQLServer database (maybe 4 or 5 forms in each accept input; the rest are just used to review data in different formats).

I've been aware of the need for this for some time, so I have only a few global variables, and most of the forms could easily be modified to use private data sessions.

So, what else will I need to be concerned with? I ain't never done no non-modal application before, and I assume there are common pitfalls to avoid (um, the first of which is to design it as modal in the first place, but it's a little late for that one ...)

Any pointers will be appreciated,

Thanks in advance