I was trying to downgrade my local mongodb version from 3.4 to 2.6.11 in ubuntu 14.04, I'm getting the following error when I'm trying to start mongodb:

2017-01-27T10:24:16.190+0600 [initandlisten] exception in initAndListen: 28574 Cannot start server. Detected data files in /var/lib/mongodb created by storage engine 'wiredTiger'. The configured storage engine is 'mmapv1'., terminating

Here is the procedure I've followed

remove existing mongodb version

sudo apt-get --purge remove mongodb-org mongodb-org-*
sudo apt-get --purge autoremove

install 2.6.11 version

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
echo 'deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen' | sudo tee /etc/apt/sources.list.d/mongodb.list
sudo apt-get update
sudo apt-get install -y mongodb-org
sudo apt-get install -y --force-yes mongodb-org=2.6.11 mongodb-org-server=2.6.11 mongodb-org-shell=2.6.11 mongodb-org-mongos=2.6.11 mongodb-org-tools=2.6.11

pin version

echo "mongodb-org hold" | sudo dpkg --set-selections
echo "mongodb-org-server hold" | sudo dpkg --set-selections
echo "mongodb-org-shell hold" | sudo dpkg --set-selections
echo "mongodb-org-mongos hold" | sudo dpkg --set-selections
echo "mongodb-org-tools hold" | sudo dpkg --set-selections

start mongodb

sudo service mongod start

debugging

cat /var/log/mongodb/mongod.log

2017-01-27T10:24:16.190+0600 [initandlisten] exception in initAndListen: 28574 Cannot start server. Detected data files in /var/lib/mongodb created by storage engine 'wiredTiger'. The configured storage engine is 'mmapv1'., terminating

screenshot