Delegating copy constructor and const data initialization

I have a class A with lots of data members, some of which are constant. All data members have proper copy constructors, so I want to default a copy constructor of my class:

class A
{
public:
        A() : a(1) {}
        A(const A& op) = default;
private:
        // ... Lots of constant and non-constant member data ...
        const int a;
};

Then, I want to write a constructor which accepts a reference to A and a value which should initialize one of constant data members:

A(const A& op, const int a_);

Here op should be copied, and a should be initialized with a_ after that or instead of copying. I would like to avoid manual initialization of all data members by delegating to a copy constructor, but how to overwrite my const data member in this case? For example:

// Illegal: constructor delegation can't be mixed with field initialization.
A(const A& op, const int a_) : A(op), a(a_) {}

// Illegal: attempt to assign constant member.
A(const A& op, const int a_) : A(op) { a = a_; } 

// Hack. May lead to UB in some cases.
A(const A& op, const int a_) : A(op)
{
    *(const_cast<int*>(&a)) = a_;
    // ... or same tricks with memcpy ...
}

Obviously, all of these approaches are evil as they try to initialize a twice.

Another solution is to move all constant data to a base class and write all needed ctors, but it looks to verbose.

Is there a cleaner way to implement A(const A&, int a_)?