This is really no problem, because bullets are designed for this action. BUT... they are NOT designed to endure this action repeatedly. Each time you chamber a round, you are putting pressure on the bullet and, ever so slightly, adjusting its position in the casing. How many times can you do this with a round before it creates a misfire? The answer is dependent on each individual round.
The policy I trained my officers to follow was to ALWAYS replace the round that had been in the chamber. In other words, if you are going to change from duty ammunition to practice rounds at the range, get rid of the round that was previously in your chamber. You could fire it for practice while doing your training. If you are cleaning your firearm, you can toss that chambered round into a specially designated box for practice later. Either way, you should NEVER rely on that round again.
A good rule to follow is to replace all of your defense rounds once a year. Your ammunition in your magazine would probably last ten years or more without any problems. But are you willing to bet your life on it?