Program not working as intended

[t0ny]

@blh64

const int interruptPin = 3;
#define MOTOR_EN_1_2  10
#define MOTOR_IN1     9
#define MOTOR_IN2     8

const int cspeed = 150;// Constant Speed
int currentSpeed;


enum { STATIONARY, SLOWDOWN, ACCELERATE, COAST };
int currentState;

volatile byte flag;

unsigned long currentTime = 0;
unsigned long previousTime = 0;


//*************************************************
void setup()
{
  Serial.begin(9600);
  Serial.println("Motor Speed Up Hold Slow Down with interrupt");
  pinMode(interruptPin, INPUT_PULLUP);
  pinMode(MOTOR_EN_1_2, OUTPUT);
  pinMode(MOTOR_IN1, OUTPUT);
  pinMode(MOTOR_IN2, OUTPUT);
  attachInterrupt(digitalPinToInterrupt(interruptPin), change, CHANGE);
  Accelerate();
}

void change() {
  // interrupt service routine
  flag = 1;
}

//*********************************************
void Stationary()//Motor stopped for interval time
{
  const unsigned long interval = 5000;

  if (currentTime - previousTime >= interval)
  {
    currentState = ACCELERATE;
  }
}

//*********************************************
void SlowDown()
{
  const unsigned long interval = 400;
  static int idx = cspeed;

  digitalWrite(MOTOR_IN1, HIGH);
  digitalWrite(MOTOR_IN2, LOW);

  if (currentTime - previousTime >= interval)
  {
    previousTime = currentTime;
    analogWrite(MOTOR_EN_1_2, currentSpeed);
    Serial.println(currentSpeed);
    currentSpeed--;
    if ( currentSpeed == 0) {
      // we are done decelerating so transition to STATIONARY
      currentState = STATIONARY;
    }
  }
}

//*********************************************
void Accelerate()
{
  const unsigned long interval = 400;

  if (currentTime - previousTime >= interval)
  {
    analogWrite(MOTOR_EN_1_2, currentSpeed);
    Serial.println(currentSpeed);
    currentSpeed++;
    if ( currentSpeed >= cspeed ) {
      // we are done accelerating so transition to COAST
      currentState = COAST;
    }
  }
}

//**********************************************
void Coast()
{
  // nothing to do since we are up to speed
}

//***********************************************
void loop()
{
  currentTime = millis();

  int oldState = currentState;
  
  if ( flag ) {
    currentState = SLOWDOWN;
    flag = 0;
  }

  if ( oldState != currentState ) {
    beginState( currentState );
  }

  checkState( currentState );
}

void beginState( int state ) {

  currentState = state;
  previousTime = currentTime;

  // do whatever you need to start this state
  switch (state) {
    case STATIONARY:
      digitalWrite(MOTOR_IN1, LOW);
      digitalWrite(MOTOR_IN2, LOW);
      Serial.println("Stopped");
      break;

    case SLOWDOWN:
      digitalWrite(MOTOR_IN1, HIGH);
      digitalWrite(MOTOR_IN2, LOW);
      Serial.println("Slowing down");
      break;

    case ACCELERATE:
      digitalWrite(MOTOR_IN1, HIGH);
      digitalWrite(MOTOR_IN2, LOW);
      Serial.println("Speeding up");
      if ( currentSpeed < 50 ) currentSpeed = 50;
      break;

    case COAST:
      Serial.println("at speed");
      break;
  }
}

void checkState( int state ) {

  // do whatever you need to during this state
  switch (state) {
    case STATIONARY:
      Stationary();
      break;

    case SLOWDOWN:
      SlowDown();
      break;

    case ACCELERATE:
      Accelerate();
      break;

    case COAST:
      Coast();
      break;
  }
}

This is much more than I expected. Thank you very much.