Almost always no. What you have today is a fairly incremental process that also guarantees the changes being made are well-localized, and thus can be reviewed

Almost always no. What you have today is a fairly incremental process that also guarantees the changes being made are well-localized, and thus can be reviewed to ensure that overall business logic is not affected. The other issue is that most likely, you don't have anyone who really knows how these applications work (otherwise, you probably wouldn't have been running them on "old versions of SunOS"). This means that even if you had the luxury of a rewrite, it's an open-ended problem with no set deadline. So enjoy massaging the code into something modern. When you're done, you should have a good understanding of both design and implementation issues, and you can make the decision for a rewrite then (but most likely, the answer will be a 'no' again...).