If one of my bottom front teeth moved forward after wearing braces and retainers, is there a way to correct only that tooth? and if the root of that tooth is already short, is there a way to correct the tooth without making its root shorter?

Yes: Have your orthodontist have a look & give you options. May be as simple as wearing your retainer.

Shifting teeth: Retainers need be worn for as longs you want your teeth to stay straight. Once discontinued, teeth may shift. A tooth can be moved if ther is enough room. If adjacent teeth have drifted into displaced tooth's space, then more than just correcting one tooth is required. Please see a fully qualified Orthodontic Specialist to evaluate and advise, including condition of tooth root.

Complex Questions: If only that one tooth shifted (highly unlikely) then only that one tooth will be repositioned. Moving teeth with light, continuous forces should not damage or cause the root of that tooth to resorb. Your local orthodontist can further explain.