Is there's any risk using modern C++ compiler on DAL A Display Application Flight Software?
The issue with many things in flight safety is that the status quo is well understood. Changing something introduces a whole body of unknowns, and considerable circumspection will be applied by the people charged with ensuring safety. This is why piston engines in many light planes are almost identical to those used half a century ago - they're known to work - and why there is considerable inertia when it comes to updating your software tools.

Ultimately, the only way things are likely to move is when somebody builds a case for updating the tools that shows sufficient commercial benefit to overcome the additional cost of certifying the final product. 

It won't be enough to simply argue that a new compiler has this or that set of new features. You'll need to demonstrate that using the new compiler will allow you to provide a set of new features that you can't provide with the old one, and that this will lead to a tangible increase in sales.

Good luck!