Licensing concerns in porting S-Plus code to R libraries

I used S-Plus for many years, and developed lots of code for my research.
When I switched to R a few years ago, I ported most of my S-Plus code over
to R for my own use. Now that some of this code is quite polished, I would
like to make it available as an R package on CRAN.

Some of my S-Plus code, however, was based on Insightful S-Plus functions;
for instance, I took a Trellis function, and modified to suit my needs. In
some cases the modifications are minor (e.g., adding extra functionality,
changing colors), while in other cases I've made substantial changes (e.g.,
a new function that just uses the techniques of the internal function).
Furthermore, in porting the code to R, I of course had to make further
changes (e.g., Trellis functions for lattice/grid functions, new C code to
replace internal S-Plus routines to which I did not have the source code).

I wondering if anyone can provide some guidance here as to what I can
release without running afoul of any user agreement that may have come with
S-Plus long ago. I'm assuming other package developers, especially those
that were S-Plus users long ago, have encountered this issue and can advise
me on what to do.