Want to test how well you know a tune? Move it to a different place on your instrument. Specifically, move the tune to a different octave.
Shifting octaves is one of the simplest ways to vary a tune, but it's not necessarily easy to execute! Like everything else, it must be practiced.
Select any tune you know. Play it as you learned it. Now, try playing it an octave higher. Can it actually be accomplished on your instrument? Do you have all the necessary notes? If no, well... good to know! Return to the original location of the tune. If yes, practice until the tune is as easy to play in the upper octave as it is in the original octave.
Now, return to that original location on your instrument. Ahhhh.... feeling better?
Next, play the tune an octave lower. Same question: Can it be done? If no, you're finished. If yes, practice until you can play it as smoothly as in the first setting.
Even if the entire tune can't be played in the upper or lower octave, sometimes the A part works, or the B part, or a portion of one or both parts. Try it all and take note. This is valuable information that allows you to take full advantage of the range of your instrument for any given tune.
Once you know what can be done, play around with it. Play the entire tune in one octave, then jump to a different octave. Or move by parts ... play part A in one octave, part B in another. Or A1 in one octave and A2 in another. Or move by phrases. Or whatever. Just have fun!
The real fun comes when you begin to find ways to shift from one octave to another using bits of scales and pieces of chord arpeggios. The sky's the limit!
Warning: Don't make the mistake of playing the tune in a different key! That's fun, too, but not what we're doing here.
Having trouble knowing where to start? Identify the beginning note of the tune. What's its name? Move to the note with the same name an octave higher (an interval of 8, all inclusive). Don't forget that this is not a linear instrument. You will not be moving straight up the instrument to find your new note!
When moving to the upper octave you will cross the bridge or valley to the left, looking up the instrument for the new note in the same relative position to the mark as the original note. If the original note is on the mark, the new note should be, too. If the original note is one above the mark, the new note will be, too. If the original note is one below the mark, the new note will be one below the mark.
When moving to the lower octave, you will cross the bridge or valley to the right, looking down the instrument for the same-named note that is in the same relative position to the mark as the original note.
Sure does take a lot of words to describe this! Much easier to show it on the instrument. Good luck! and let me know how it goes :-)