Do you really need to pre-plan your character because you cannot redo your choices made after a skill up ? Or can you redo previous choices ?

Generally, most people plan some perks to do their builds so they don't waste any, as it is difficult to level up later. But this can be avoided with the new DLC: Dragonborn. Don't know how to use spoiler yet so I can't say, but I bet someone knows how. But anyways, just have fun. And if you've got the DLC, you'll be able to mend any errors you make.

On PC you can use the console to add and remove perks - there are likely mods to do it, as well.

Well if do not have the dragonborn DLC then no you cannot refund your perks. But if you have the dragonborn DLC then you can read the black book "walking dreams" and it will take you to the realm of hermaus mora where all your perks will be displayed and can click on them to refund them.