There is an upgrade on the classic Global's with CRT's (DU870) to LCD (DU875) displays although it is expensive quotes run from $600-900K. u.s. depending on when the modification is done (stand alone or during a 60/120 month inspection) there has been quite a bit of push back from operators regarding Honeywells decision to not support them after 2018 so a solution may still appear. There is now a retrofit to put the LCD units in if a CRT fails although you will not get the functionality of CRT (charts on screen, better graphics etc.)
Let me know if you have any further questions on the Global, can't help with Gulfstream.
Cheers
MB