When changing the domain you send from in Mailgun, it's a good idea to copy the existing suppression list to the new domain to prevent spikes in bounces and complaints.

mailgun_migrate_suppression_list.sh

#!/bin/bash
if [ -z "$1" ]
  then
    echo "Missing API Key argument!"
    exit 1
fi

if [ -z "$2" ]
  then
    echo "Missing source domain!"
    exit 1
fi

if [ -z "$3" ]
  then
    echo "Missing destination domain!"
    exit 1
fi

APIKEY="api:$1"

echo "Processing Bounces"
NEXT_URL="https://api.mailgun.net/v3/$2/bounces?limit=100"

while [[ ! -z "$NEXT_URL" ]]; do
  echo "Importing $NEXT_URL..."
  RESULTS=$(curl -s --user "$APIKEY" -G "$NEXT_URL")

  NEXT_URL=$(echo $RESULTS | jq -r '.paging.next')
  ITEMS=$(echo $RESULTS | jq '.items')
  if [[ "$ITEMS" = "[]" ]]
    then
      break
  fi

  curl --user "$APIKEY" --header "Content-Type: application/json" --request POST --data "$ITEMS" "https://api.mailgun.net/v3/$3/bounces"
done

echo "Processing Complaints"
NEXT_URL="https://api.mailgun.net/v3/$2/complaints?limit=100"

while [[ ! -z "$NEXT_URL" ]]; do
  echo "Importing $NEXT_URL..."
  RESULTS=$(curl -s --user "$APIKEY" -G "$NEXT_URL")

  NEXT_URL=$(echo $RESULTS | jq -r '.paging.next')
  ITEMS=$(echo $RESULTS | jq '.items')
  if [[ "$ITEMS" = "[]" ]]
    then
      break
  fi

  curl --user "$APIKEY" --header "Content-Type: application/json" --request POST --data "$ITEMS" "https://api.mailgun.net/v3/$3/complaints"
done

echo "Processing Unsubscribes"
NEXT_URL="https://api.mailgun.net/v3/$2/unsubscribes?limit=100"

while [[ ! -z "$NEXT_URL" ]]; do
  echo "Importing $NEXT_URL..."
  RESULTS=$(curl -s --user "$APIKEY" -G "$NEXT_URL")

  NEXT_URL=$(echo $RESULTS | jq -r '.paging.next')
  ITEMS=$(echo $RESULTS | jq '.items')
  if [[ "$ITEMS" = "[]" ]]
    then
      break
  fi

  curl --user "$APIKEY" --header "Content-Type: application/json" --request POST --data "$ITEMS" "https://api.mailgun.net/v3/$3/unsubscribes"
done

echo "Finished!"

Requires jq and curl.