Seo Will not be effected by just ticking the box to expand globally. What will hurt you is not using the proper use of the href lang tags to the proper pages.
For example if you are targeting an italian audience and your href lang tag is saying it is in english and it leads to an italian page written in italian. This will cause ranking issues on the US version of Google but your seo will not be effected by pages currently ranked in the United States.
You can not be dinged for duplicate content in different languages as google has different searches for just about every country and the ranking algorithms run for that specific country. Each language will be ranked according to that demographic.
Also your content should be in the language you are trying to target. For example a user in brazil will never see websites from the United States unless they change their language settings in the search engine settings to english. Or the pages you have on your website are written in the specific language for that region.
An American English Website even if it is targeting users globally will not rank as well as websites written and developed in the local region of that user in their country. You can read more about it here from google. https://support.google.com/webmasters/answer/182192?hl=en <-- this is exactly what you are looking for and will answer you question more in depth.
Before ticking the the change of reach in Webmaster ensure your website is built for Multi-Region use. As it won't effect your United States ranking the site will never rank in the global searches until you can tailor pages specifically to your users.
Most users will do a subdomain of their website and launch each country separately. For example fr.yourwebsite.com and all the content on this subdomain is written specifically for france. Or bz.yourwebsite.com for customers in brazil. You can just tick it on and expect your english website will rank in other places around the globe.