I, and everyone else I see, pray witr after taraweeh. I am not sure if it is suggested or obligatory. If I don't pray taraweeh until I pray witr, and later I decide I want to pray taraweeh, can I do that?
The Prophet (saws) said:
"Make witr as your last prayer at night." Sahih al-Bukhari 998
There are many other ahadith similar to this. Based on this the scholars say that it is recommended for the last prayer that a Muslim prays in the night be witr. This command is interpreted in the sense of recommendation because there is another hadith that the Prophet (saws) prayed two raka'at after witr sometimes as well, and Imam an-Nawawi says that he did so in order to show that it is indeed permissible to pray after witr.
Finally, IslamQA (run by Shaykh Munajjid) says:
So if you want to pray Tahajjud at night, it is permissible for you to pray Witr with the congregation, then to pray after that as many rak’ahs as you want two by two, and do not repeat Witr.
In summary, it is permissible for you to continue offering tahajjud or taraweeh after witr, and recommended and mustahabb to pray witr last when you can. However, make sure not to pray witr twice, since the Prophet (saws) also said that there are no two witrs in one night.