If the change is minimal your image will remain uniformly bright regardless of lens position. Some lenses are now manufactured using lightweight plastics. If weight is not an issue, try to find a machine with an all-glass lens. Glass is the optimal filter for projecting images and will give you a clearer picture. However, glass lenses do make a projector heavier, so pick plastic if you are going to be on the road a lot.
Optional lenses are also available through some manufacturers and resellers.
If your conference room machine will be called upon to project at different positions, angles, and brightnesses, you should ask about interchangeable lenses. Keep in mind the market is extremely competitive and prices continue to drop each year. A general, and obvious, rule to follow is the more features you want the more expensive the projector. Keep in mind that prices do vary among dealers, so it usually pays to shop around. Additionally, some manufacturers may give special pricing to their top dealers, which can translate to larger discounts for you.