Recently my iPhone was stolen, and instead of putting it in lost mode, I erased it via Find My iPhone.
Is there any way to switch it to Lost Mode? Can I see its last location? And will the thieves be able to use my iPhone now?
You will be able to locate an erased iPhone if it's near a previously used Wi-Fi network. The thieves won't be able to use your iPhone as well.
Quoting from the Apple Support document, Find My iPhone: Erase your device:
When you remotely erase a device, Activation Lock remains on to protect it. Your Apple ID and password are required to reactivate the device.
Important: Before you erase your device, try to locate it or play a sound on it. After you erase it, you can’t use Find My iPhone to do either. You may still be able to locate your Mac or Apple Watch if it’s near a previously used Wi-Fi network.