If I were to switch to Mycelium mobile wallet, I can just download the app from App store and key in my 12 seed phrases and it works? Or do I need to do or change something on my Electrum wallet first before I switch to Mycelium forever?
There is no easy way to import your Electrum wallet into Mycelium, since both seeds are not compatible with each other. If possible, I suggest you to just move all your coins to a new Mycelium wallet and start using it for your new transactions.