Since web-centric software development is newer than client-server, there have been fewer examples of this type of transition to study. In general, all the points discussed for mainframe to client-server transitions above hold true. Here is an additional point to consider for this type of transition:
Many software experts have commented that web-centric software applications really herald back to the mainframe days as now both data and applications are being re-centralized into larger servers. The water is further muddied as IBM and others release Java Virtual Machines, web server software, and other web-centric applications for allowing one to actually implement a web-centric design on a mainframe. Independent of hardware platform, however, the software architectures used on traditional mainframe applications and web-centric applications remain entirely different. Don't think your mainframe development staff can become instant web-centric experts simply by purchasing a web server for your mainframe.