yes you can change it many times. i did that once. actually just today i also won 2nd leg champions league which i changed to the earliest possible time after the maintenance. my opponent was from italy so he has to get up early in the morning which i assume is rather difficult for him. well the same for me actually. but i always get up early in the morning so for me this time is perfect. and it was a success. he didn't watch the game and i won it

for changing it multiple times, there could be many reasons. maybe your opponent changes his mind and decides that the previous arranged match is not good. maybe suddenly he couldn't attend the previously arranged time. or maybe he just wants to piss you off and hopes that you miss this change he makes. for this guy (moreover, i think he's a token buyer) i think you need to be careful and check your updated fixture frequently. i know it's pretty well disturbing since you must have another game and scheduled training session

but he cannot advance your match today that's for sure. he can only delay it