Will sitting for the IELTS Academic module void the validity of my previous General Training module score?
For the best information you could Contact IELTS

That said, from reading the information booklet (pages 3-5) on their website the two tests have some sections that apply to the both GT and Academic tests which are speaking and listening. They then have 2 modules that are different for each test, which are reading and writing. 

They are also used for two different purposes 


  IELTS Academic is for test takers wishing to study at undergraduate or postgraduate levels, and for those seeking professional registration.
  
  IELTS General Training is for test takers wishing to migrate to an English-speaking country (Australia, Canada, New Zealand, UK), and for those wishing to train or study at below degree level


From this it would seem unusual that the results from one test would void the other when they are both different tests. 

Edit based on OP comment:
After the OP contacted IELTS thay said that 'they are both separate results and it will be your choice entirely which one you wish to present when applying to institution's.' which confirms that one test would not void the other.