Create a .bat script with the following contents:

@echo off
setlocal enabledelayedexpansion
for %%f in (*.mp3) do (
    ffmpeg -i "%%f" -q:a 5 "%%~nf.ogg" >nul 2>&1
    if !errorlevel! == 0 del "%%f"
)

It will take all the MP3 files in the folder it's placed in, convert them to OGG and delete the originals.

It requires ffmpeg to be installed or inside the same folder if you happen to have the full .exe

Just make sure to take all non-mixtape files (.bat and .exe) from the folder before uploading it.

To Do: make a non technical guide for this