Good answer from Norman, but I would add this:
Almost certainly there is going to be some cleanup, at a minimum,
involved. If the application is complex, contains a lot of VBA code,
or in particular has complex reports, for example reports with
calculations within them, then there could be really a lot of work to
do to clean up the conversion or conversion might not even be
possible. Crystal Reports is a lot different than the Access report
module.
So it seems to me the skills required to re-write the app are similar
to those required to clean up the 3rd party conversion. After all, the
database itself will just be converted to SQL Express or perhaps
simply separated into a back end Access DB and the .net app linked to
it - something that is easy to do manually. That leaves the front end.
If it is simple, and you have those skills, then why not just re-write
it yourself as the software is probably expensive? If it is complex,
then the software is unlikely to be able complete the conversion on
it's own and you will be stuck with a useless partial conversion.
Peter