sillypenta is taking it in the right direction. To go even further, you could have body parts transform in sequence to create some anticipation for the big transformation. So for example, you could have the arm shake and then pop a new hand on, shake the other arm and then that transforms, then build it up until there is one big reveal.
When you have the minifig standing still then transform all at once, the audience is not prepared for the story beat. This could cause confusion, or it could leave the audience disinterested in that story moment. Sillypenta improved upon this by adding some shaking to anticipate the transformation, allowing the audience to develop more intrigue in the upcoming action. My suggestion above takes it further and makes the anticipation even more dramatic. Key word: anticipation!
These are just guidelines, and depending on the overall style of your video you might adopt different animation philosophies.