I think it would be foolish for you to make the change from Delta to United. You may have seen the ads by United about how their program has been voted the best program for 8 years. Well, that is their old program. Their new program is awful, especially for elite flyers. In fact, as part of their changes, United significantly slashed benefit that had been previously promised for life to their million mile flyers.
When an airline show this little integrity, one should not even expect them to keep commitments going forward, making their program completely useless.