EDIT: I just reread your post, so I want to make sure I understand. The following assumes you want to edit the Red Lion program to use its serial port to communicate with a Compactlogix serially because the Compactlogix Ethernet port has failed. If that is correct, read on. If not, please ignore the rest of this and reply with clarification.
Before you start, save the Crimson file with a new name.
Then, As Ken said, all you should have to do is create a driver for the serial port that is the same type as what you have assigned now to the Ethernet driver. Then find the PLC under your existing Ethernet driver and take note of the device name (default is PLC1).
Delete that device and you will notice at the lower right, the Error indicator will light up "red" indicating that your project has a problem, but no worries.
Create a new device or rename the newly created default device under your serial port driver to match the name you just deleted.
If the error indicator at the lower right stays red, right click on it and click "Recompile Database".
With a Compactlogix, I think that should be all you have to do aside from procuring or fabricating the right serial cable, dumping the program into the DSP and plugging it in.
There is a chance you may have to reload (Import) the L5K file...not sure since I have not done this exact task with that combination of software and hardware.
The cable you need:
Red Lion Link with docs including pinout
Allied sells it and delivers fast...but they're out of stock.
What driver is being used now and if it is the Native Tags driver, do you have the L5K file that goes with it?