I think this will depend on the treaty. If Diego Garcia remains as UK-sovereign land, then since different laws (etc) apply it's likely ISO would keep the IO c

I think this will depend on the treaty. If Diego Garcia remains as UK-sovereign land, then since different laws (etc) apply it's likely ISO would keep the IO code for it. If Mauritius keeps the islands they gain with a different status (tax, immigration and so on) compared to the rest of Mauritius, then a code might be needed for that — but Mauritius probably won't be keen on "IO". If the whole lot becomes 'ordinary Mauritius' then the code is no longer needed and will be removed.