Spellbook Improvisation

You have learnt to, on the fly, draw into your arcane readings and prepare what’s needed for the moment at hand. 

Once per short rest, as a bonus action, you can change a spell you have prepared. You must unprepare a spell you have prepared, and when you do so, you prepare a different spell that’s in your spellbook, up to a level equal to 1 + half your proficiency bonus (rounded down).