Rooflights need to be high quality and reliable. Equally, the roofs that they are fitted into need to be high quality and reliable as well, so that you can guarantee your rooflight will be safe. You can always upgrade a rooflight, but you cannot upgrade it into a poor quality roof, so you may as well make sure that it is installed into solid foundations.
When you are having a rooflight replaced or you are installing a new one, it is beneficial to check the quality of the roof first. In theory, it might be possible to install a rooflight, but in practice, this might not be the right option. Get the quality of the roof and its structural integrity checked over and make sure it can handle the weight of a rooflight. Then you can begin thinking about the actual rooflights you could install and which one will work the best for you.