Dynamically changing cut-off and sampling frequency of a digital filter
There are a few ways that you could do run-time dynamic filters.


With a program like Matlab or Octave, but I'm guessing that they aren't available to your application.  There might be DSP libraries that have functions for generating filters though.
Calculating from scratch a windowed sinc function.
You could create somewhat crude low-pass filters based off of your starting filter.  For instance, if you wanted to halve the cutoff frequency you would halve the filter sample time.  You would thus create a new filter that had all of the original filter's samples and a sample in between those samples.  You could create the extra samples through splines.  This same approach could be used to create any fractional cutoff frequency change.  A more complicated example would be if you wanted the cutoff frequency to be $\frac{9}{8}f_c$.  If we say that your original filter's samples are at time 0, 1, 2, ..., $n$, then the new filter's samples would be at time 0, $\frac{9}{8}$, $\frac{18}{8}$, ...