This is not about how to use fancy converter like mediacoder. I'll show "the hardway" to convert all of your MP3s to Ogg Vorbis using open source tools only.
I'll assume you have a quick access to command prompt from explorer. If you don't have it, try googling "Command Prompt Shell Extension" and install it.
1. download the CLI (Command Line Interface) tools here. Inside the zipped file, you'll find:
lame.exe -> Mp3 encoder version 3.98.4 (with mpg123 decoder builtin)
venc.exe ->AoTuV(Aoyumi Tuned Vorbis) encoder 2.2.2
oggd.exe -> ogg vorbis decoder 1.0.1
vorbisgain.exe -> vorbis replaygain version 0.34
tag.exe -> universal audio tag writer version 2.0.52
m2v.bat -> recursive mp3 to vorbis batch command
v2m.bat -> recursive vorbis to mp3 batch command
2. extract it to your windows folder e.g. C:\Windows or anywhere inside PATH environment
3. to convert all mp3 file in a folder and all of it's subfolder, browse into your mp3s collection folder then right click you folder and choose "Open Command Prompt". Type m2v and enter.. wait untill it finished.
Explanation about the batch file
for /R %%f in (*.mp3) do lame --quiet --decode --priority 4 "%%f" - | venc --quiet -q0 - -o "%%~pf%%~nf.tmp" && vorbisgain -q -f -s "%%~pf%%~nf.tmp" && ren "%%~pf%%~nf.tmp" "%%~nf.ogg" && tag --fromfile "%%f" "%%~pf%%~nf.ogg" && del /q /f "%%f"
Merely it says: find file with .mp3 extension in the current folder and its subfolders, convert MP3 into wave format and at the same time convert the wave into vbr 64kbps (don't worry its as crispy as 128kbps MP3) ogg vorbis file named *.tmp then if OK proceed with applying replaygain (neutralize volume) then if OK rename the temporary file into *.ogg, then if OK transfer the ID3 tag from MP3 to the newly made ogg vorbis, finally delete the MP3 file. Repeat until all MP3s gone.
After it finished, you will lose at least half space from your previous mp3
There is v2m.bat with similar (reversed) process, but you might wont use it.