Microchip do provide a parametric search facility on their web site. So do many other manufacturers.
Therefore the first step is for you to decide what the search parameters are for your application.
Simplistically, if you have a PIC18F the you may well have the IDE and compilers that are required for the newer members of the Microchip families (including some of the newer PIC16 which are pushing on the bottom end of the PIC18 family functionality, and also the PIC24/dsPIC33 devices). The development techniques will flow across the families quite well whereas changing manufacturers may involve a new development environment and coding techniques.
On the other hand, if there is some parameter that is significantly better served by another manufacturer then makign the change can be worth it.
Susan