Keep the app free, and if ad-free, keep it like that. Try to make it bigger. This is your investment in the future. If you have an app with +1 or +10 million installs, that's a beautiful item on your resumé. If you really want to use that app for money, create new functionality via in-app payments, or give all old users a life-time free version that can be transferred to other devices. Better use it to attract attention from investors and let them give money to you for a second app.