Mike - reactive lymphocytes are actively responding to an antigen rather than being damaged by something so inflamed isn't exactly the correct description although my picky-ness on this is more the result of years of arguing with physicians et al about the functions of lymphs than any real difference in terms.
The original description of these cells was "atypical" meaning having a disease (or malignancy) because the person who first saw them thought the cells were the cause of the disease he was investigating. 40 years later, it was determined that the cells were the body's defense against the disease and the name "reactive" was invented to mirror that belief. Sadly there are lots of people in the laboratory and in medicine who still call them atypical and still believe that the only time you see them is in infectious mononucleosis.
No, it is not correct that any number over the reference range is malignant. You want a large number of cells to defend you against antigenic attack. For example, it is possible to have a 100x10^9/L white cell count with 90% lymphocytes in a child with whooping cough. That is not malignant - it is "healthy" in the sense that this child's immune system is functioning correctly. What is malignant is a loss of control - these cells should respond, increase in number and in variety, and then, after successful elimination of the antigen, return to the previously seen lower number of cells.
Changes in morphology can be definitely seen in response to antigens but can also be seen in progression of lots of different lymphoproliferative diseases. I would ask if that laboratory differentiates between reactive cells and abnormal or atypical lymphocytes (as in malignant). I would also ask how the physician interprets these changes.