It depends on your reason for changing treatment. If you are switching because you have drug resistance or your treatment has not been keeping your viral load to an undetectable level, then you definitely need a new treatment. It would not be a good idea to return to your original drugs. But your doctor may be able to suggest an alternative combination to take.
However, if you are switching because of side effects, you will have more options. Sometimes people are nervous about changing treatment, worrying that their new treatment will also have side effects and that these could be more difficult to live with. This is unlikely, but if it does happen, you’ll probably be able to go back to the anti-HIV drugs you were on before.