You could status match/challenge on UA with your AA Gold status. United will give you the Premier level and you'll have to fly a certain number of UA metal miles in 3 months. Premier will then be good through next year. Even if you had lifetime Exec Plat, UA only matches up to Premier Executive.
What routes will you be flying? Based on your profile it looks like you're in the ORD area, which will remain a very large hub after the merger.