How do you figure? It's similar to the 3.x ability for the Sorcerer to occasionally swap out a known spell at certain levels. Also, Bo9S had a mechanic where at odd levels the 3 classes gained new abilities, while at even levels they could swap out an old ability for a different one. That was combined with a series of strikes within each discipline that were obviously upgraded versions of lower level ones, thereby allowing you to keep your core skills and powers relevant to your current level.
As far as Safeguard Smite is concerned, I could easily see a more powerful variant a few levels later that would apply the AC bonus to the entire party; an option to upgrade via power swap simply makes sense. It still doesn't lock out the choices made through gaining totally new powers (and having to choose among them); instead it helps solidify and reinforce the role that you've been building up throughout your adventuring career.