I am sure there is a better way to do this, my client only had a few links to fix so I got away with a dirty bit of SQL but a better SQL could be written to find all of the MultiUrlPickers who's value is not updated from RelatedLinks. I.e. has a caption field etc.

I created a migration so it's only ran once.

var sql = Sql($"SELECT upd.* FROM dbo.umbracoPropertyData AS upd INNER JOIN dbo.umbracoContentVersion AS ucv ON ucv.id = upd.versionId WHERE upd.textValue LIKE '%\"caption\":%' AND upd.textValue LIKE '%\"isInternal\":%' AND ucv.[current] = 1");
        var dataToConvert = Database.Fetch<PropertyDataDto>(sql);

        if (dataToConvert.Any())
        {
            Logger.Info<NestedContentRelatedLinkMigration>("Found " + dataToConvert.Count + " RelatedLinks to fix");

            foreach (var propertyDataDto in dataToConvert)
            {
                var items = JArray.Parse(propertyDataDto.TextValue);
                if (items.Any())
                {
                    foreach (var item in items)
                    {
                        if (item is JObject arrayProperty)
                        {
                            var aliasFound = LinkAliases.Where(x => arrayProperty.ContainsKey(x)).ToList();
                            if (aliasFound.Any())
                            {
                                foreach (var alias in aliasFound)
                                {
                                    var i = arrayProperty[alias];
                                    var t = JsonConvert.DeserializeObject<List<RelatedLinkDto>>(i.ToString());

                                    var toLink = (from x in t
                                        select new LinkDto()
                                        {
                                            Name = x.Caption,
                                            Target = x.NewWindow ? "_blank" : null,
                                            Udi = x.Link.Contains("umb://") ? x.Link : null,
                                            Url = x.IsInternal == false ? x.Link : null
                                        }).ToList();

                                    arrayProperty[alias] = JToken.Parse(JsonConvert.SerializeObject(toLink));
                                }
                            }
                        }
                    }

                    propertyDataDto.TextValue = items.ToString();
                    Database.Update(propertyDataDto);
                }
            }
        }