"no change to the substance"

I thought that the time extension from the supreme court allows me ,in addition to correcting the petition form, to also improve on the content of the petition but have just noticed this line in the Clerk's letter:


"When making the required corrections to a petition, no change to the substance of the petition may be made".


The rules of the court does not seem to get into that. It is confusing and unexpected because how can the change to a petition be measured starting from first accepted submission? 
The objective could be not to allow changes altering the petition to a fundamentally new one. In other words the purpose could be to prevent changes at a level similar to that which would not prevent a re-filing of the case because of the like of the res judicata rule if it were a case at a district court. 
If that interpretation is correct then the way they stated it was unnecessarily costly to me because it probably would force me to resend the original petition with all its substance shortcomings.   
What also supports this interpretation is that it doesn't seem very probable that the court would treat differently in that regard extension based on what is judged as a failure after a good faith effort without asking for extension for the original filing period, from extending the period after approving such request.