Just to note that the 45 days does get "used" no matter which version you install, and once it runs out, it has run out for all versions (older or newer). When a new version comes out (the final release of the new version), the trial does get an extra 14 days to try it out, so you would be able to test Corona 7 for 14 days once it releases.
To be honest at this point in time, there are no major changes between 6 and the 7 daily build, the majority of the new features are not in there yet, so you wouldn't learn any more than during your trial of 6 (which is why we only renew the trial on release of the final, because then there really is new stuff to try out, and in working form too ;) )