Configuration Settings of 18F4550
If you just want to run the '4550 the same way as the '452, then use the same oscillator settings.  Apparently you were using a 20 MHz crystal, which would have been without the PLL since that would have made it out of spec.  You can do the same with the '4550.  Use the HS oscillator mode without PLL.  That part has a more complicated clock chain due to needing to support the USB peripheral, but you can make it run the same way as a '452 from the same crystal.

Why switch to the '4550 if you're not going to use its new features?  The older '452 is still in production and likely will be for quite a while.  Microchip has a very good track record not obsoleting parts.

These chips have a hardware UART.  Why do it in firmware?

Even if you are using a soft UART, you should be able to change the clock speed, change a assembly constant accordingly, and have the soft UART code configure itself to the new clock speed automatically.

Too many things don't make sense in your question.