The earliest available dates are: end of September for IELTS, and end of October for TEF/TCF or DELF/DALF. :'(
On the other hand, there's this information on their page:
Will I be able to modify my online application once it has been submitted?
After you submit your application, you will receive a message in your mailbox informing you of when it will be possible for you to modify your application online.
Upon receipt of this message, you will be able to:
add or remove a spouse or dependent child;
add new information on your stay in Québec;
add one or more language tests;
modify information on the members of your family residing in Québec.
If you are informed that your application is not being processed, you can return to your account at any time to update your form.
I can still give it a shot, right?