If you just want to change the value (without a special editor) back and forth between the two formats, you just need a custom TypeConverter. Declare the property like this:

public class MyClass
{
    ...

    [TypeConverter(typeof(MyStringConverter))]
    public string MyProp { get; set; }

    ...
}

And here is a sample TypeConverter:

public class MyStringConverter : TypeConverter
{
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return sourceType == typeof(string) || base.CanConvertFrom(context, sourceType);
    }

    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
    {
        return destinationType == typeof(string) || base.CanConvertTo(context, destinationType);
    }

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        string svalue = value as string;
        if (svalue != null)
            return RemoveSpaceAndLowerFirst(svalue);

        return base.ConvertFrom(context, culture, value);
    }

    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        string svalue = value as string;
        if (svalue != null)
            return RemoveUnderscoreAndUpperFirst(svalue);

        return base.ConvertTo(context, culture, value, destinationType);
    }

    private static string RemoveSpaceAndLowerFirst(string s)
    {
        // do your format conversion here
    }

    private static string RemoveUnderscoreAndUpperFirst(string s)
    {
        // do your format conversion here
    }
}