Pierce gets paid $16,790,345 this year. That amount is fully guaranteed.
Next season, Pierce's deal is worth a reported $15,333,334. Last night, Pierce played in his 41st game, which triggers a clause in his contract that guarantees him $5 million. The remaining $10 million+ becomes fully guaranteed if Pierce isn't waived by July 1, 2013.
I suppose it's theoretically possible that the team could waive Pierce, pay him his $5 million, and then sign him to a new contract. We'd lose Bird rights to him by doing so, though, and could only sign him using cap space or another exception (like the MLE).