This will be the fourth time in my career where I will be at a large library system changing over the cataloging/circulation database.
When I was at Broward County after 1994 and before 2003, it was when they switched from a Web Cat to a CARL (I think it was Dialog @ CARL, but it's been so long...). It was better networked, more menu-driven, more GUI friendly.
When I was at the University of Florida Libraries from 2003 to 2006 it went from WebLUIS to ExLibris / Aleph, which provided more coordination with online databases, a proto version of Integrated Library Systems (ILS).
When I was at Pasco County Libraries from 2006 to 2008 it went from CARL (with a DOS-style text-only variant for circ staff, compared to the Internet-friendly OPAC screen) to CARL.X, with a more ILS and keyword-friendly format.
Here at Bartow Public Library, as part of the Polk County Library Cooperative we're switching from SirsiDynix Horizon to Symphony, the company's ILS solution.
Timing for me is apparently everything. I bet if the Library of Congress hires me they'll decide a few months later to switch over from LC to another cataloging system altogether.
Why the switch?
Each change in the systems were due to changes in technology. GUI or Graphic User Interfaces weren't really feasible until the mid-90s when the operating systems improved to be more GUI oriented (Win 95, anyone?), hence a lot of library catalogs were text-based commands rather than selected commands up until 1996 or so. With the spread of the Internet came more online databases and the need to integrate journal and magazine and newspaper articles into search results, needing the ILS formats from 2003 onward. With the advent of e-books and digital printing, the need to integrate e-book collections in OverDrive and NetLibrary alongside the print catalog is a must, which is where Symphony is the choice PCLC is going with.
Next up will probably be further integration into non-traditional resources, such as video and audio streaming. Just think of having to keyword link various menu selections to The Avengers DVD/Blu-Ray...