Is it permissible to change the intention of the Shaf' prayer to Witr during the prayer?

It is permissible for someone who prays three rak'ahs for Witr to pray them consecutively, or to say the taslim after two rak'ahs and then pray the third. If the worshipper intended the even number of rak'ahs from the Witr, then what the Imam did does not harm him, and there is no need to renew the intention, because the intention is already present. However, if the follower did not intend that the two rak'ahs were part of Witr, then later learned that the Imam was praying Witr, he may renew the intention for Witr without interrupting the prayer, or he may intend to separate from the Imam and say the taslim, then join him again with the intention of Witr.