Create a new collection with the old indexes

mongo.js

var i, c, 
    co_name, co_new, 
    co_old, co_old_i, 
    _db = db.getSiblingDB('logs'), 
    _colls = ['activity.games', 'activity.session', 'activity.social', 'activity.store'];

for (i in _colls){
    co_name = _colls[i];
    co_old = _db[co_name];
    co_old_i = co_old.getIndexes();
    if(co_old.count() > 0){
        co_old.renameCollection(co_name + '.old');
        _db.createCollection(co_name);  
        co_new = _db[co_name];
        for(c in co_old_i){
            co_new.ensureIndex(co_old_i[c].key);
        }   
    }
}