There is a FAQ for Windows Audacity Manual but Mac is usually more forgiving and flexible about installing programs.
There are no concerns that I am aware of. 2.0.3 uses the same project format as 1.3.12, so there should be no problem opening projects in 2.0.3 that were created in 1.3.12.
Your current settings from 1.3.12 will be used for 2.0.3 where that setting still exists in 2.0.3. Although you can reset your audacity.cfg settings file manually so that 2.0.3 starts out with its own “as shipped” defaults, there is no need to do so. Please see http://manual.audacityteam.org/o/man/preferences.html#stored .
Most Mac users would simply trash the /Applications/Audacity folder for the old version they were replacing. In your case you will need to move your AUP files and _data folders out of there, along with any exported audio files, before trashing.
Similarly if you have stored optional plug-ins in the 1.3.12 Audacity’s “Plug-ins” folder, you will need to move those out before trashing 1.3.12.