I have a standard Data Extension as the entry audience of the Journey.
I would like that everytime the value of a specific text field changes - the Contact will be injected into the Journey.
Is it possible?
Kind of. It's a bit tricky.
Journey Builder does not detect existing record changes in the Data Extension, nor does it detect replacing a record in your Data Extension. This is known as the High Water Mark issue.
If an import or SQL activity is updating records in the event source data extension, they will not be assigned a new row in the data extension. This means that for contacts who may now meet filter criteria for the trigger, but who did not on previous runs of the entry event, they will still not be injected since they fall below the High Water Mark.
In short, the last row in your DE is under the High Water Mark and if an event is fired, only records added after that are injected to the Journey.
If you want customers to be re-evaluated each time the Event is fired, then you will need to create a SQL activity or Data Filter (with Automation) to target the Journey source Data Extension and overwrite the Data Extension totally, which then essentially resets the Water Mark.