The changes are that the data you are receiving is not straight characters, but binary values. This may be because the device you are connected to is not sending string data but binary values, or it could be because your port configuration is incorrect: baud rate, bits per character, or parity for example.
Try reading the data from the port as bytes, and displaying the hex values:
Dim i As Integer = SerialPort1.BytesToRead
Dim data As Byte() = New Byte(i - 1) {}
i = SerialPort1.Read(data, 0, i)
Dim s As String = String.Join(" ", data.[Select](Function(c) c.ToString("X02")))
That should at least give you an idea what data is being returned.