Learn Spanish (Latin America): Francamente, el cambio drástico de perspectiva sorprendió a todos. - Frankly, the drastic change of perspective surprised everyone.

No, here it modifies the entire sentence rather than the verb. As a speaker attitude marker, it tells you the speaker's stance (that they are being frank or honest) about the whole situation. These markers usually sit at the very beginning of the sentence and are set off by a comma.

Why do we need the a in sorprendió a todos?

This is the personal a. Whenever the direct object of a verb is a person or a group of people, you must put an a in front of it. Even though todos (everyone) is a general pronoun rather than a specific name, it still refers to people, so the rule applies.

Could we put the adjective first and say el drástico cambio?

You could, but it changes the flavor of the sentence. Putting drástico after the noun is the standard, objective way to describe the change. Moving it before the noun would make the sentence sound more subjective, literary, or emotional.

Why use the preterite sorprendió instead of the imperfect sorprendía?

The preterite sorprendió treats the reaction as a single, completed event: the exact moment the change was revealed, everyone was surprised. If you used sorprendía, it would sound like an ongoing background state, such as 'the change was in the continuous process of surprising everyone'.