Hola guille, puedes hacer algo similar a lo siguiente:
Código vb:
Ver originalPrivate Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim ctl As Control
For Each ctl In Me.Controls
If TypeOf ctl Is TextBox Then
ctl.Text = ctl.Text.Replace(".", ",")
End If
Next
End Sub
Saludos!