Depends on the shape and size of the existing dwarf walls Jane. Usually easier to knock them down and start again as its imperitive that the walls are in exactly the right place to sit any conservatory on.
We are currently in the middle of this. We had a wooden conservatory (that was here when we bought the house) and it was leaking and not very well put up in the first place.
We took the wooden top part down ourselves (and have just had the builders in the re-do the walls - these were not proffessionally built and you could tell - one wall was an inch further out at the top compared to the bottom.
You can take down a wooden conservatory and errect a UPVC one in its place but if it is a P shape it needs to have a return of at least 300mm which wasn't required with wooden ones. All you need to do is take external measurements of your existing basework, take them to your local supplier where they will make the necessary reductions to calculate internal frame sizes needed for production. One other thing to check is the house wall, weather it is plumb or not. It can lean in, out or have a wave appearance. Again, telling your conservatory supplier this info, he should be able to make needed adjustments to the roof size and or add add ons to your frames where they meet the wall which can be scribed to suit the wall profile.