It's easy to do but require some skill with the mission editor. You need to locate the escort ship and them add a tug with the same speed, date and with the same waypoints exactly over the old one (to avoid AI problems). After you do that, just select the old one and delete it. It's also possible to do this in a text editor but I think that this is a more difficult way.
If it's to hard to you just contact me after I release the Rub version (I'm busy with it now) and I will do it for you.
Rubini.