The words :فَوقَ،تَحْتَ ،قَبْلُ ، بَعْدُ are special mudhafs.
If the mudhaf ilaih is present then they are fully flexible like we see in
مِنْ قَبْلِكُمْ and مِنْ قَبْلِهِ
But many a times the mudhaf ilaih is omitted – then they become non flexible-
they are just fixed to their original harakah then – like dhamma with قَبْلُ and fathah with تَحْتَ
So we see it as:
مِنْ قَبْلُ
as the mudhaf ilaihi is omitted.
Motivated Sisters 