Clients recently purchased a home and realized the balusters may pose a danger to their children due to the spacing of the balusters. The age of the house was such that they should have been to code - but weren't.
If a child can stick their head through the balusters it is not to code. It is definitely a safety issue and should be replaced.
That said – if you plan to sell your home and your balusters are over 4 inches apart you should consider replacing them to make it easier to sell your home to a family with small children – Updating your Stair will only bring value to your home and worth doing.