Fixing creaky floors in attic conversion

Hi all. We've bought a house that previously had the attic converted into livable space (3 rooms and a WC). The floor boards in the converted attic space sound very creaky from downstairs when someone's walking around. We plan to change all the flooring upstairs anyway so we're thinking of solving the issue now while we're at it. Any ideas of how to fix this - or has anyone else solved a similar issue? Any ideas about the costing involved too? Thanks!