Changing or removing command linked to a button after clicking - python-3.x

Question

Playing around with Tkinter and ran into some trouble.
I have 3 buttons:
Button1 will open my specified website using Selenium.
Button3 will carry out my function on the website.
Button2 will close the program.
Problem: I want to keep button1 from opening multiple windows/websites by
either removing/hiding it after it has been clicked or changing the
command
that will be performed by it.
After some googling I found some options and tried the following:
| pack_forget()
| pack_destroy()
| btn1.destroy()
All of the above does not seem to be doing anything. What am I missing?
import selenium
from selenium import webdriver
from time import sleep
import time
import tkinter
from tkinter import *
main = Tk()
def Primary():
txt = mtxt.get()
New = Label(main, text=""+txt, fg='Blue').pack(side=BOTTOM)
btn1 = Button(Bot, text='OK', command=Primary,
fg='Green').pack_destroy() #For some reason this does not work
return
def Secondary():
global main
main.destroy()
def Tres():
txt = mtxt.get()
New = Label(main, text=""+txt, fg='Blue').pack(side=BOTTOM)
return
main.geometry('350x400+500+300')
main.title('Title')
mtxt = StringVar()
Top = Frame().pack()
Bot = Frame().pack(side=BOTTOM)
Lbl = Label(main, text='TEXT').pack()
Lbl2 = Label(main, text='TEXT').pack()
Enter = Entry(main, textvariable=mtxt).pack()
btn1 = Button(Bot, text='OK', command=Primary, fg='Green').pack()
btn2 = Button(Bot, text='Cancel', command=Secondary,
fg='Red').pack(side=BOTTOM)
btn3 = Button(Bot, text='Next', command=Tres, fg='Green').pack()
main.mainloop()
Expected result:
On initial click btn1 performs 'command=Primary'
After initial click btn1 performs 'command=Tres'
or
after initial click btn1 is destroyed/hidden and btn3 takes it's place.