I have some new LED headlamps I would like to use on my 2003 S-type. Can I just replace the existing bulbs with these or is there anything else required?

 

If they are the correct ones (ie H7 for dip), they are simply plug and play. I did mine, dip and main beam, about 18 months ago, they've been trouble free with much improved lighting. It's a fiddly job fitting the bulbs as space is limited.