Here is a simple function and some example code that will modify the value of a string var to lower case. Just pass the stringvar to the function.
from tkinter import *
root = Tk()
#No need for the following line in your code
root.withdraw()
def lowerStringVar(var):
"""Function to convert the text in a StringVar to lower case"""
if isinstance(var, StringVar):
var.set(var.get().lower())
myTextVar = StringVar()
myTextVar.set("ALL UPPER CASE")
print(myTextVar.get())
lowerStringVar(myTextVar)
print(myTextVar.get())