Now.

Whatever & however you spend you time practicing is what you are going to be good at.  If eventually you want to be good at standing & playing with a low slung strap, then you are going to have to spend a lot of time doing that until it is second nature.  Certainly the conceptual aspect can be learned sitting or standing.  The notes are all in the same place, the chords & scales are all the same shapes.

But if you are only used to doing all your playing sitting with a guitar in your lap, then there is going to be a learning curve transferring & adapting all your knowledge to the physical technique of playing in a different position.

This is also why I mention transitioning to electric ASAP.  It's a different physical machine.

Hope that helps.