Match from United 1K to Emirates Silver successful in September 2018. Emirates will only bump up by one tier, no matter what status you're trying to match from. This is the second time in about 5 years that I have done it successfully - from Platinum to Silver the first time, and now 1K to Silver. They do have a cooling off period of a few years, but I'm not sure what it is. Emirates Silver gets you lounge access in Dubai, and that alone is worth the effort.
It can be hard to find a direct contact - this e-mail address worked for me: email@example.com
Create an Skywards account first. Provide as much material as you can - proof of current status, recent statements showing travel, etc. - and have an explanation for the amount of travel you have planned. May also help to have an actual booking in the system. I had a 4 leg trip from the Middle East to the US on the books when I started the process, and plans for 3 more multi-leg trips, including long hauls, over the next several months.
It can take several weeks of back and forth by e-mail (they say 4-6 weeks), along with a phone call or two to a representative who actually handles the membership tiers. I was a call back by phone from a membership representative that eventually sealed the deal. Be polite and persistent!