In order to downgrade to Mavericks from Yosemite do I need to reformat my hard drive and use the internet recovery to reinstall Mavericks?
As of today there is no known way to downgrade from Yosemite to Mavericks without erasing the drive.

Booting to the internet recovery mode altcmdR means that you boot to a netboot image containing a basic system downloaded from Apple/Akamai. You don't need the Mavericks installer. If you reinstall a system the system originally shipped  with your Mac will be loaded.


Boot process (takes 2 minutes connected to a 50 MBps-line):  

 
Boot process finished:  

  

Translation:
Festplattendienstprogramm: Disk Utility
OS X erneut installieren: Reinstall OS X
Starting Disk Utility looks like this:  

  

Now you just have to repartition your Macintosh HD: 1 Partition &amp; GUID partition table (available via Options)
If you stuck here unable to partition your drive because it's forbidden or greyed out, your Macintosh HD was probably converted to a CoreStorage Volume Group. Please check and execute the steps 10-12 of my answer to this question. Afterwards you have to start Disk Utility again and partition the drive to your needs.
BTW: SystemUSB is an attached thumb drive with a bootable system and disk2 (OS X Base System) is the netboot image you downloaded and booted to earlier in step 1.
Quit Disk Utility to get to the menu again (picture 2)
Start Reinstall OS X. Now probably Lion is downloaded to your newly created hard disk partition and installed there afterwards.
If you get a message like 'Could not find installation information for this machine' just restart your Mac to the internet recovery mode. (The netboot image seems to be very picky with the WLAN connection and it took me two attempts to get around this step)


If you choose the recovery mode cmdR the latest system will be installed. You need a working Recovery HD-volume though. Such volume won't exist on a brand new hdd.

In both cases a stable and fast internet connection will be convenient.

Using the internet recovery mode you won't end up with an 'unbootable' system (depending on the reliability of your internet connection) because your (maybe falsely partioned) hdd is never involved in the (net)boot process.

In your case (MacBook Pro early 2011) i would download 'Mavericks' with your App Store account, create a 'Mavericks Installer' thumb drive, boot to it and continue with step 2 outlined above. It's just faster and more reliable than first downloading and installing Lion and then Mavericks.