The Last name "Mack"

My grandfather on my fathers side said that his father had his last name changed to Mack when he came to the U.S. I'm just wondering what it could have been before that, since my grandfather never told us before he died.

Possibly, the surname was originally Macek.

two other possibilities; Mackowski or Mackiewicz

These are some but not all of the Polish surnames someone may have shortened to Mack:
Mackała Mackałło Mackało Mackan Mackaniak Mackanić Mackaniec Mackanis Mackań Mackaszwili Mackay Macki Mackiel Mackiełło Mackier
Mackiera Mackiewicz Mackin Macko Mackos Mackoś Mackowski Macków Mackuj
Mackuła Mackun Mackus Mackusz Mackuś

Maybe they added the letter 'c'. The word 'Mack' sounds similar to 'Mak' in Polish.
It means poppy.

Nazwisko/last name