53 Pa. Stat. § 102
The classification of said cities respectively, shall be ascertained and fixed by reference to the last two preceding United States decennial censuses, and whenever it shall appear by both of such censuses, that any city of the first, second or second A class has decreased in population below the minimum population figures prescribed for its current classification, or that any city of the second, second A or third class has increased above the maximum population figure prescribed for its current classification, it shall be the duty of the Governor, under the great seal of the Commonwealth, to certify the fact accordingly, which certificate shall be entered at large upon the minutes of the councils of such city and recorded in the office for recording the deeds of the proper county. No change in classification or in the existing form of government shall become effective until ten years after the certification of the fact of decrease or increase of population by the Governor: Provided, That the court of quarter sessions of the county in which the city is located shall appoint a charter commission to study and make recommendations on the adoption of a form of city government under sections 3, 4, 5, 6 and 7 of this act, and a form of government is adopted within the above ten year period. If a form of government is not adopted within the ten year period, the city shall automatically become classified according to the provisions of section 1 of this act and be subject to the provisions therein.
It is the intent of the preceding paragraph that the classification of any city of the first, second, second class A and third class shall not be changed because its population has changed at the time of one United States decennial census, because it is recognized that a change in the form of municipal government is attended by certain expense and hardship and such change should not be occasioned by a temporary fluctuation in population but rather only after it is demonstrated by two censuses that the population of a city has remained below the minimum figure or above the maximum figure of its class for at least a decade.
At the municipal election occurring not less than one month after the date of such certificate the proper officers shall be elected to which the said city will become entitled under the change in classification, and upon the first Monday of April next ensuing the terms of all officers of said city then in office whose offices are superseded by reason thereof shall cease and determine, and the city government shall be duly organized and shall thereafter be controlled and regulated by the laws of this Commonwealth applicable to the same under the classification hereby fixed and appointed.
Until otherwise provided by law, cities of the second class A shall continue to be governed, and shall have all the powers, privileges and prerogatives now provided by the laws of the Commonwealth relating to cities of the second class.
53 P.S. § 102