Creating a new Loft Conversion sometimes involves inserting a Steel Beam or two at floor level as well as one at roof level.
If you did put in bigger joists then these will have to be taken out and re inserted into the new beam/s when you have a Loft built.
The insulation cannot be re used as the one needed for a Loft would be a rigid type 'tuff board' (celotex or similar) and not the one used within floor joists in Attics.
