As it's not guaranteed that the triggering tag will be updated anywhere, anywhen (meaning even if the tag is not programmed on the current screen), you need to declare the "upfate attribute" as "Cyclic continuous"

best rgds

E=mC² (Errors =MoreCode²) :-)

Look at the steps to do this:

1- Create a tag of type Int ( In this example the name of the tag is “screen”)
2- Select “screen”, go to properties>General>Acquisition mode>Cyclic continous
3- Select “screen”, properties>Events>Change value>select “ActivateScreenbyNumber”> on the field “ScreenNumber”, select the “screen” tag.

Operation: When the tag "screen" change of value (the PLC) to screen the same number is call on HMI.
(Note: To check / set the number of each screen in WinCC Flexible, select the screen from the main menu,> Right click> Properties> "Number".

Important Note: If the choice of screen is controlled by the PLC and the operator, sequence above should be complemented by an additional tag (binary), which should also change the screen using the tag "screen" as a reference (PLC as well as putting the value of desired display the tag "screen", the tag must reverse torque to force the exchange)