Is it permissible to change the intention of the Witr prayer from three continuous rak'ahs with one tashahhud to two rak'ahs followed by a separate rak'ah, whether out of forgetfulness or intentionally?

Both joining (consecutive performance) and separating (performing with pauses) in the Witr prayer are valid, but separating is preferred. Changing the intention from one to the other is not permissible, as it would result in a deficiency or an excess compared to what was initially intended. Therefore, when one intends a specific number of units for a prayer other than an absolute supererogatory prayer (nafl mutlaq), it is not permissible to decrease or increase that number. If someone who intended to perform Witr consecutively deliberately says the salam and performs the tashahhud after two rak'ahs, and then intends to convert them into an absolute supererogatory prayer before his salam, those two rak'ahs will be counted for him as an absolute supererogatory prayer. Then, he should begin the shaf' and Witr with the intention of either joining or separating, and adhere to what he intended. If he did this out of forgetfulness, he should stand up to complete the remainder of his prayer if the interruption was brief, and perform the prostration of forgetfulness (sujud al-sahw). If the interruption was prolonged, he must repeat the entire prayer.