Examples:
Emperor Domitian was reviled by the Senate and historians of that time as a mad tyrant but some now consider him just a brutal but effective emperor
or how barbarians were hated for extinguishing the light of Rome at first but were then romanticized into freedom fighters rising up against their oppressors.
What causes these changes in opinion? Is it the discovery of new information? Is it people of a particular time trying to put past events into a modern context? Both?
Emperor Domitian [...] but some now consider him just a brutal but effective emperor
Who does that?
barbarians were hated for extinguishing the light of Rome at first but were then romanticized into freedom fighters rising up against their oppressors
They are completely different sets of barbarians, but it would make it easier to answer this part if you could show which peoples (barbarians) you've been looking at, as a barbarian essentially is anyone not living on Roman lands.