I agree with Daryk. If there are rules for moving between ranges, there should be a rule for moving out of range. In theory, you could handwave a rule that any unit already at long range can expend X movement to leave long range, but this really highlights the problem with vague rules for combat.
A rules-lite roleplaying system works fine (IMO). You don’t need a skill for every action or a rule for every interaction. Destiny may not have a specific skill for acting in an off-broadway play, but you can pretty reasonably use Charisma plus something else. The sample characters in the book are short, but they provide all of the detail I need to get a feel for how these people are supposed to act.
Combat is another story, though. Even as a theater of the mind game, not explicitly defining ranges or movement in quantitative measurements opens up the door to a metric ton of confusion and arguments. The Beta lays the groundwork for a really good, mini-Battletech. The armor and damage are already reduced and simplified. If they re-worked the movement and combat ranges (both character and mech) into those same simplified statistics instead of simplified suggestions of statistics, I think the combat would be much more solid.