Where any partnership agreement to which the last preceding section applies is subsequently modified or is replaced by a subsequent agreement, the agreement as so modified or, as the case may be, the new agreement shall, so long as at least two of the partners to whom the agreement relates were partners under the original agreement on and immediately before the appointed day, be deemed for the purposes of the last preceding section to be the original agreement, and any subsequent modification or replacement shall be treated in like manner so long as the condition aforesaid remains satisfied:
Provided that the last preceding section shall only apply in relation to the purchase, and rights and obligations in respect of the purchase, of a share in the goodwill of a medical partnership carried on under any such agreement, if the persons who are or would be parties to the purchase were members of the partnership on and immediately before the appointed day and the provisions of the agreement relating to the purchase are substantially the same as they were immediately before the appointed day.