C#

public static class TransformationUtilities
{
    public static long GetCheckInteger(long longValue)
    {
        // decompose longValue into two lists of digits

        // do the right thing
        // using 'TransformDigit, and 'SumOfDigits
    }

    // note: auto-conversion from digit to long here
    // low priority: examine how efficient that is
    public static long TransformDigit(int digit)
    {
        if(digit < 5 ) return digit;
        return SumOfDigits(digit * 2);
    }

    public static long SumOfDigits(long value)
    {
        // conversion to 'string expensive
        // find a better way
        return (value.ToString()).Sum(ch => ch - 48);
    }
}

return (value.ToString()).Sum(ch => ch - '0');

public const long TenBase = 10L;

// 12345678 => {8,7,6,5,4,3,2,1}
// note that IEnumerable Type must match parameter Type
// but you can pass Integers of any integer Type as the parameter value
// without error
public IEnumerable<long> IntToDigitList(long longint)
{
    while (longint > 0)
    {
        yield return (longint % TenBase);
        longint /= TenBase;
    }
}