Does Boeing have a statute that commands that the windscreens are changed for new irrespective of no signs of damage or fatigue, and if so, what are the intervals, for safety reasons?
First, Boeing does not provide any statues for when maintenance is performed. They provide guidelines and recommendations through the Maintenance Planning Data (MPD) manual that most airlines follow. Some of these recommendations are required to be performed by the FAA with the certification of the aircraft or later AD action.
That said, there are no recommended maintenance tasks for the 787 windshields. The reasons is windshields are certified to be able to to hold cabin pressurization even with every pane shattered. Although there is no recommended or required task to replace the windshields at a set interval, some airlines may choose to do so for operational reasons.
Mandatory part replacements based on flight hours or cycles is almost exclusively reserved for a special classification of hardware known as "Safe Life Parts" these are generally just the Landing Gear and Engine Rotors. These are required to be scraped at specific intervals due their failure mechanisms and because they can be single point catastrophic failures.
I have specifically reviewed whether or not hard time replacement of 737/757/767 and 777 windshields made sense at a very large airline. The data showed that windshield failure rates when plotted against either cycles or hours had very large standard deviations. This means it is very hard to set an appropriate hard time interval, if you set it too early you would have few failures in the field but you replaced a lot of windows that had a ton of life left on them, if you set it too late you still had many field failures and were still throwing away good windows. When doing a Wiebull analysis I was able to identify at least 8 different failure mechanisms that all had very different rates and only affected a small population of windshields. To highlight the point, we had multiple windshields that failed after less than 100 flight hours while we had many that lasted more than 60,000 flight hours (and a few that were up to 80,000 that had never been replaced).