Stain Over Existing Stain on Vertical Surfaces

Hello. It's my first time in this site a few hours ago and I've learned a lot already from all the articles the site has.

My wife and I are in the middle of fixing our deck, replacing some boards and patching the existing ones.  My question is more on the balusters and posts.  Since vertical surfaces do not deal with the same wear and tear, is it still necessary for us to strip them all down?  We are planning on using a darker solid stain for everything.  We live in Mount Olive, NJ.

Thank you, all.