I have been practicing the classic Elliott Wave theory and want to switch to the modified "Harmonic" version. How much time it could take me unlearn the traditional theory and pick up the new one ?

First of all we are talking about two versions of the same Elliott Wave theory. Both versions have the same foundation and differ only in details. Moreover, corrective waves are treated in the same way by both versions. So you only need to switch from looking for micro five waves to counting three wave a-b-c moves.

To trade confidently you not only need to learn the theory but you you should keep applying it in practical trading by yourself. That is an essential stage required in automation skills of recognizing patterns, measuring waves and finding confluence of projections and retracements to identify high probability target boxes

If you are experienced in the traditional Elliott Wave theory it could take you not more than three to six months to become proficient in applying the Harmonic Elliott Wave theory in your daily trading life.