update Task.WhoId to ConvertedContactId for invisible Tasks related to converted Leads (Shared Activities enabled)

gistfile1.txt

Map<Id, Lead> ls = new Map<id, lead>([select Id, ConvertedAccountId, ConvertedContactId from Lead where IsConverted = true]);

Task[] ts = [select WhoId, WhatId, (select Id from TaskRelations) from Task where WhoId in :ls.keySet()];
system.debug(ts.size());

TaskRelation[] deletes = new TaskRelation[]{};
TaskRelation[] inserts = new TaskRelation[]{};
for (Task t : ts) {
	for (TaskRelation tr : t.TaskRelations) {
		deletes.add(tr);
	}
	TaskRelation tr = new TaskRelation();
	tr.TaskId = t.Id;
	tr.RelationId = ls.get(t.WhoId).ConvertedContactId;
	inserts.add(tr);
}

delete deletes;
insert inserts;