If the reason you are staying at B6 is truly in the hope that B6 will be bought out, you are STILL rolling the dice. Ask the VX guys at AS - those ones still there. You never know what the SLI is going to look like, or what the retirement situation will be at the combined carrier.
United still hasn't hit peak retirement and that covers a multitude of issues, but - yeah - no question it's starting over on probation pay.