Bottom line:  There is not much evidence available on this question.  Most recommendations are based on expert opinion.

A search of PubMed and CINAHL for these concepts–tracheotomy, tracheostomy, ventilator-associated pneumonia, prevention–revealed one study specifically examining care of the tracheostomy–Eid RC, et al.  Successful prevention of tracheostomy associated pneumonia in step-down units.  Am J Infect Control. 2011 Aug;39(6):500-5.
Intervention included “drainage and discarding of condensate” in the tubing at least 3 times per day, but this doesn’t say specifically changing the tubing. No additional studies identified that evaluated care of the tracheostomy in preventing outcome of ventilator-associated pneumonia.

Tracheostomy: Stoma Care.  Joanna Briggs Institute, 2010.  States that evidence regarding tracheostomy is mostly based on expert opinion as there are not many published studies on tracheostomy procedures and care.

Tracheostomy:  Management – references guideline that relies on expert opinion in recommendation

Cites this small study that found no statistically significant difference in bacterial colonization between patients who had canula changed daily versus those who did not.

Burns SM, et al.  Are frequent inner cannula changes necessary?: A pilot study.  Heart Lung. 1998 Jan-Feb;27(1):58-62.
This small study that found no statistically significant difference in bacterial colonization between patients who had canula changed daily versus those who did not.  Did not look at ventilator-associated pneumonia rates.