In short, yes. You can put a solid roof on an existing conservatory.
This is called a ‘retrofit’ within the industry and is becoming increasingly popular amongst homeowners who have older conservatories and are looking to upgrade (as well as add value to their home).
It could be that your existing polycarbonate roof is leaking or that it’s just starting to look dated. Another common complaint of older conservatories is that they are ‘too cold in winter’ and ‘too warm in the summer’.
A solid tiled roof will eliminate all these issues, making the room an ‘all year round’ room.