The previous event association will link your TeamRaiser to an old event, but there is a separate setting you have to turn on to also allow teams from the previous event to be used in this event.
  1. When editing the TeamRaiser, go to step 3 "Select Event Options"
  2. On the left side towards the bottom under "Related Actions" click on "Advanced Event Options"
  3. On the left under step 3 you will see more steps appear, click on step c "Define Returning Team Options". Note that this option will not show up if you have not setup a previous event association on step 1.
  4. Check the box next to "Yes, enable teams to return from the previous TeamRaiser event".
  5. You will then have some options regarding specifics, such as the team page, team captain, etc. Once you are done click "Save"
This will allow team captains to bring back old teams for use in the new TeamRaiser.

Something also to note is that teams have to participate in consecutive events in order to properly return each year.  If a team skips a year, they will not be able to return the year after. 

For example: Team Justice participated in the 2014 Walkathon.  The next TeamRaiser they sign up for is the 2016 Walkathon.  They will not be able to bring this team back because they did not participate in the 2015 associated event.  This breaks the link between associated events for that team.