that rather depends on where they are in the bathroom, the nature of your ceiling and the type of lighting you want. this will govern wether you need IP rated (against water ingress) lights, wether you need to reinstate existing standards of fire protection etc etc.
it's not as simple as just buying some new lights and banging them in. Also, unless you are doing a like for like replacement this work will be notifiable under part P as the bathroom is a special location and as such needs to be done by a competent person who can register the works for you, or you need to notify building control before work commences.
All this basically means you need an electrician, so he will come and do a free survey (if he won't don't use him!) and you can discuss your lighting options with him then. He should also check wether you have main bonding and supplementary bonding if it is needed (probably, unless you've had a new DB in the last year or so) and wether your current installation is suitable for the work you have in mind.
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