Indeed, changing positions after performing obligatory prayers is a Prophetic Sunnah. In addition, it is a clear sign that distinguish obligatory prayer from the Sunnah that is performed after it.
Sheikh Sayyed Mutawalli Ad-Darsh, former chairman of the UK Shari`ah Council, states that: “Changing positions for performing Sunnah prayers is a sound tradition of the Prophet, peace and blessings be upon him. Just as in the case of Tarawih prayers, the Prophet always wanted to make a clear distinction between what was obligatory and what was optional. Therefore in this instance, he ordered the believers to change places, so that it did not look as if the Sunnah prayers were an extension of the Fard or obligatory ones.
Thus, the object of changing places was to make a deliberate break in between the two. This is also why believers are asked to perform Tasbih or praising Allah in between the Fard and Sunnah prayers.
There is a sound tradition from the time of the Prophet relating an incident where a believer – having finished his Fard prayers – immediately stood up to offer his Sunnah. Omar, upon the Prophet’s notice, ordered the man to sit down and wait.
With so much emphasis on Fard prayers being offered in the Mosque, the Prophet advised that Sunnah prayers be performed at home in order to provide a break and also to ensure that peoples’ homes are never devoid of worship. In this context, the Prophet, peace and blessings be upon him, is reported to have said, “Do not turn your houses into graveyards.”