Like this:
List<long> longs = ints.ConvertAll(i => (long)i);
This uses C# 3.0 lambda expressions; if you're using C# 2.0 in VS 2005, you'll need to write
List<long> longs = ints.ConvertAll<int, long>(
delegate(int i) { return (long)i; }
);
List<int> ints = new List<int>();
List<long> longs = ints.Select(i => (long)i).ToList();
var longs = ints.Cast<long>().ToList();