Set it up like you set up your AT or how you wished you could have if you could change something. You can notch or disc out what worked for you before. You can set the tones to the same breakpoints and frequencies or add another tone if you like. If you liked standard mode, you can set it up like that. If you liked pro mode better it is already set up more like that. If you like two tone, leave it as is. If you like to hear iron, use disc A or G. If not use the default. The default is already set the minute you turn it on, two tone, no iron sound. The manual will give you all the default settings. The detector will not go back to default when you shut if off, it will save any setting you change. You can do a factory reset to get back to default settings but it will wipe out any changes you have made in all the other modes too.
I don't think there is much of a learning curve to operate the Legend successfully, just turn it on and go will work. It is the same learning curve with any detector: just observe what it is trying to tell you and, find weak targets to experiment on, to find the absolute best settings for you, just like you have done on the AT Pro.
The best advice is get out and use it for a couple of hundred hours. What works best for others won't necessarily work best for you.