I don't see anything wrong with planning a character out early - but it needs to have room for change if situations change.
For example, if your first level is rogue and, after a couple sessions of playing, the whole career path of rogue into whatever PrC you chose just doesn't look good, there's nothing that says "I must stick to what I planned!"
Just like you planned to study physics and discovered it just wasn't what you were looking for, your characters can have plans that change. You still originally planned to be a physicist, just like a character may have planned to grow up and be a Incantatrix only to discover that magic just wasn't fun for them.