Answer:
It is hard to grasp your mind around it but you need these requirements to sound correct:
1.) The strings must be replaced.
2.) The viola must be the same size of the expected violin.
3.) Must be tuned correctly (obviously)
As example, you want a size 13 inches violin and you only have the violin strings and a viola with the size of 13 inches,it will work (Ive done it before).