Is it permissible to change the intention of a voluntary prayer from two rak'ahs to four rak'ahs if the worshiper forgets the first tashahhud, or must one complete the two rak'ahs with sujud as-sahw and tasleem?

It is permissible to change the intention in a supererogatory prayer by increasing or decreasing (the number of rak'ahs), provided that the change occurs before the increase or decrease. However, if you add an extra rak'ah forgetfully, then sit down and choose between two options: The first: recite the tashahhud, perform the prostrations of forgetfulness (sujud al-sahw), and then conclude the prayer with salams. The second: intend to add as many rak'ahs as you wish after sitting, then stand up to perform what you intended to add, and perform the prostrations of forgetfulness at the end of the prayer.