Until the end of the 90's the RO desalination technology for medium and large capacities was based mainly upon two configurations of membrane modules: spiral-wound elements and hollow fiber bundles (HF). The retirement of Permasep (R) HF (DuPont) leaving HOLLOSEP (R) (Toyobo) as being the only alternative for the direct replacement of Permasep, brought a challenge to the makers of spiral elements in the sense of system retrofit to convert existing HF membrane plants into some of the most advanced spiral-wound membrane plants. The major criteria for success was to achieve substantial improvements in the economy and quality of the permeate produced at limited capital expenditure. Such improvements have become feasible with the considerable development of spiral-wound membranes in the recent years. In this paper the technical changes needed for the retrofitting have been systematically studied. After a short historical review, the paper discusses basic options for two existing Permasep installations on the Canary Islands and in the Middle East, respectively. Specific characteristics, advantages and costs of various possible solutions are presented. Two case studies are discussed for the 3x4,700 m(3)/d seawater RO plant in Agragua on the Canary Islands and for the 4x4,200 m(3)/d seawater RO plant in Gela in Sicily. Plant performance data for 8 months and 19 months respectively is presented.