It is possible to upgrade to any higher tier pledge by just paying the difference.
The forums say you joined on June 15 but don't tell me which pledge you have. You can find that out by scrolling to the bottom of this page and looking on the right side for the "subscription" link. That will take you to a page that shows your pledge level.
If you decide to upgrade, be sure to use a link that says 'upgrade' because if you just go to the link to buy a pledge, the forums will happily sell you one at full price without checking to see if you are upgrading.
When I click on the subscription link is states this:
The plan you are currently subscribed to is: Player [Read Only Access]
You are currently paying: Free
So can you please let me know what I just paid the $50 US for? What has that gotten me? Do I need to pay another $50 then to be a Legate level?
First, the Watcher pledge is $50. The Legate pledge is $100. If you have only paid $50 so far, then yes getting the Legate pledge will mean upgrading for another $50.
Your profile says that you joined the forums on June 15. Is that when you paid $50 or did you pledge in the last few days?