You can, but be aware that this can cause some problems due to the complex systems on the c5. Some problems to note include issues with the central locking, the new player being powered up for apparently no reason by the car and a few others.
I have replaced mine with a JVC unit, which produced none of the reported locking problems, but does get powered on by the car at random times. I'm told Citroen can re-program the car to ignore the replacement stereo, but I get around this by removing the fascia from the player whenever i'm not using it.
Replacing the unit will not damage your ECU.
You may find you need to swap the +12v constant and +12v ignition-controlled wires to stop the new cd player losing its settings. You may also need a gender-changer to attach the car's aerial to your new player. You will require a fascia-adapter as the radio holds in the trim for the buttons beside it(I had to do all these things - cost £15). The new player's display will not be dimmed when the button is pressed to turn all the dash lights off.
You may also like to know that you can replace all the c5's awful factory speakers with 16.5cm speakers really easily and these will fit behind the factory speaker grilles so as not to advertise your system to thieves. Don't forget to remove the tweeters in the dashboard if you wish to replace the speakers. A standard 4-speaker player will run the tweeters as they are cabled in with the front speakers, but they may sound pretty bad when you over-drive them with your new player.
Extra control units are available to allow you to use your steering-column controls with certain stereos (check the manual) though these are pricey for what they do.
Hope this helps.
andy b, April 2008
