I just comped from United to BA without any trouble. I worked through the local sales office (SEA) and it was very easy. I simply faxed them proof of my UA status (Prem. Ex.) and gave them some idea of my near term travel plans and they comped me to BA gold. The only hitch in the process is that it takes time to get my new status in the system so I will not get the status bonus for my trip next week. The sales rep is preparing a letter of introduction for me though so that I can use the gold check-in and lounges.