You can’t teach old dogs new tricks.  Is it true?
 
Well, I have found that teaching adults new skills isn’t too difficult.  The difficulty comes in changing an old habit.  Changing an awkward grip, changing a stroke, or changing a footwork movement takes time and persistence from both the coach and player.  On the flip side, a new serve or a new serve return technique or a new looping variation can be learned fairly quickly for these same players.
 
If you are an adult and have been playing for more than 5 years, then realize that making a major change could be difficult.  Evaluate which weaknesses absolutely need to be changed and which weaknesses can be overlooked by developing a different shot.  In order to move up 100-200 points this year, try to make minor changes on your weak points and focus the bulk of your training on developing new weapons that can compensate for your weak points.