I would approach this differently. I would create a Global Variable called "Speed" and set its value to e.g 400.
Then i'll create a condition saying "every tick" > set playersprite bullet speed to ""Speed". (It should get its speed value from the global variable we created.)
Now you can create multiple objects or whatever and link their bullet speed to the global speed variable. Now whenever you want to change the speed. Just create a condition and set the 'speed' variable value to something else. This way you can change the speed of multiple objects at once without any trouble.
hope this helps you.