How to keep upward momentum after reversing gravity

I am currently working on a small 2D game with a gravity shifting mechanic and I have come acros a small problem which I hope someone could be able to help me with.

The concept is rather simple, I have an object moving along the X axis and it has two functions - a Jump and a Shift. Jump is simple enough, and as for Shift, what it does is changes the players Y position to the oposite and reverses the gravity - up is down and down is up.

The problem comes with jumping and shifting at the same time, as I shift while jumping the object reverses the gravity and looses all of its upward momentum and plumets down/up.

What I would like to achieve is the ability to keep the upward/downward momentum after a shift.

Here is the code I have writen thus far for both of the functions:

     public void Jump()
     {
         if ((isGrounded && rb.velocity.y <= 0 && rb.velocity.y >= 0))
         {
             jumpsAvailable = 1;
         }
 
         if (jumpsAvailable > 0)
         {
             canJump = true;
         }
         else
         {
             canJump = false;
         }
 
         if (Input.GetKeyDown(KeyCode.UpArrow) && canJump && isUp)
         {
             rb.velocity = Vector2.up * jumpForce;
             jumpsAvailable--;
         }
         else if (Input.GetKeyDown(KeyCode.UpArrow) && canJump && !isUp)
         {
             rb.velocity = Vector2.down * jumpForce;
             jumpsAvailable--;
         }
 
     }
 
 
     public void Shift()
     {
         if (Input.GetKeyDown(KeyCode.Space) && isUp)
         {
             if (!isGrounded)
             {
                 transform.position = transform.position + new Vector3(0,
                 (-transform.position.y * 2)
              + (-transform.localScale.y + 0.074294f), 0);
                 rb.gravityScale = -3;
                 isUp = !isUp;
             }
             else
             {
                 transform.position = transform.position 
             + (new Vector3(0, -transform.localScale.y + 0.074294f, 0));
                 rb.gravityScale = -3;
                 isUp = !isUp;
             }
         }
 
         else if (Input.GetKeyDown(KeyCode.Space) && !isUp)
         {
             if (!isGrounded)
             {
                 transform.position = transform.position + (new Vector3(0,
                 (-transform.position.y * 2) 
                - transform.localScale.y - 0.03899369f, 0));
                 rb.gravityScale = 3;
                 isUp = true;
             }
             else
             {
                 transform.position = transform.position +
                 (new Vector3(0, transform.localScale.y - 0.03899369f, 0));
                 rb.gravityScale = 3;
                 isUp = true;
             }
         }
     }

Curently the only solution I was able to think of was to delay the gravity shift while there is still upward/downward momentum, but I am not certain that it would be the optimal solution, so I thought that it would be better to try and get an answer here.