vb.net有关的问题

如果把大写字母换成小写字母，小写字母换成大写字母，是用命令按钮的单击事件来完成，应该用什么语句？小弟只弄到只能把大的变小，不能把小的变大
怎样能弄成这样的效果：I Love You换成i lOVE yOU，应该怎么写才对，我想表达这样的意思

假设两个textbox,textbox1放转换前的string,textbox2放转换后的string

按button1后执行

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim strBefore, strAfter, strCurr As String
'strBefore:转换前的string
'strAfter 转换后的string
'strCurr 当前处理的string

strBefore = Me.TextBox1.Text.Trim

For i As Integer = 0 To strBefore.Length - 1
strCurr = strBefore.Substring(i, 1) '取第i位字符
If strCurr >= "a" And strCurr <= "z" Then '小写字符,转为大写
strCurr = strCurr.ToUpper
Else
If strCurr >= "A" And strCurr <= "Z" Then '大写字符,转为小写
strCurr = strCurr.ToLower
End If
End If

strAfter = strAfter & strCurr
Next