Power on with new circuits but with old program

What happens if I power on the arduino with new circuits on my breadboard, while the old program is still on the Arduino?

I'm new to the world of electronic circuits, microcontrollers in general and the Arduino. Maybe my questions sounds stupid, but I have to ask - I'm worried I could damage my Arduino.

The thing is... I set sth up on my breadboard, connect the Arduino pins, write some code in the IDE and upload it to the Arudino. Works fine. But then I disconnect the Arduino from my computer, change the stuff on my breadboard, write some new code in the IDE and then I have to upload the new program to the Arudino. To do so, i have to connect the USB cable again. The old program starts - with the new stuff on my breadboard. - This could damage sth I think. So I better disconnect all the Arduino pins before I do so... but if I have to do so... everytime... it gets really annoying.

So I ask: Isn't there a better way? Maybe starting the Arudino without a program, so nothing happens - or deleting the old one somehow? What do you recommend? - Or maybe, am I worrying to much? :D

Thanks in advance for your comments.

Nice greetings from Austria! Christoph

A good practice is to program the Arduino with "blink" or "minimum sketch" before starting a new circuit.

Oh my god... why didn't I think of that? This should be the easiest way! Thank you! ;)