Just ask them nicely. Happened to me last year when I bought my house, and to be honest, I don't think it was cos they were planning to gazump me. House had gone sale agreed twice already, had been on sale for two years, and buyers couldn't come up with cash, so I could see their point. Plus most EAs have a skeleton staff on now, and taking down sale agreed signs is not on their priority list.
Still, I asked them kindly to do it, and within a week, they had it changed to sold. It took them about 2 months to take down the sold sign after I moved in though!
If they think you are feeling insecure or about to pull out for any reason, I'm sure they'll be jumping all over you to keep you happy! Ball is in your court remember!