No Spoiler Review :
I know a lot of fans are furious & confused with the sheer amount of changes made in the events & date of events in this movie as compared to its predecessors, but rest assured that it's only because this movie is a soft reboot & not because the new director, Travis Knight is ignorant about the storyline set by the previous movies. This movie is still produced by Michael Bay itself, if not directed by him.
They made the necessary changes because they want to create a range of better movies; & that is because the storyline set by the previous movies had pretty much come to an end, the franchise was going downward spiral in terms of box office & also the ratings, & the Last Knight lost hundred million dollars against the budget even though it made around 600 million at the box office.
Though I enjoyed the previous movies, I must admit that they were really kinda messy & looked stretched; despite having better action, the story telling wasn't sharp; even a YouTube video summary of the movies by IGN could do a better job in this regard & you'd be surprised on how much in depth you missed. Since a better story telling was required, Bumblebee was created as an experiment, which would serve as a bridge between part prequel & part reboot.
If the movie is to be successful, they will fully reboot the franchise; but if the movie doesn't succeed, then they will continue the previous storyline as it is, & all fans have to do is ignore some of the changes made in this movie; but it looks like it's winning at the box office & ratings, both, & also because the budget for this movie is lower than usual. So, if you find yourself questioning why Bumblebee is on Earth in 1989 instead of already being here & serving in the WW2 among other things, know that it's all intentional & relevant to the plot that's coming ahead for good. They are just retconning.