I have a 1.3 Terrabyte music collection and 8000 of those songs are in .mwa format. I want to convert to mp3, however the converter I have only does one at a time which soon will become tedious

Is there a converter that will search the drive, convert, and replace the wma automatically?

Handbrake

^ Handbrake is a VIDEO transcoder. It doesn't accept audio files as input. Besides, transcoding from one lossy format to another (.wma to .mp3) isn't recommended and will result in a degradation of quality. Better to leave your WMA music collection as it is.

dBpoweramp Music Converter should do it. It's supports multi core CPUs so is pretty quick. I have the older v13.5 reference version. It's my main ripping tool but I use the conversion quite a lot too, it's a great tool just not free. It looks like you'll get a full function 21 day trial which should been enough time for getting your conversion done.

http://www.dbpoweramp.com/dmc.htm