I was able to update my 17 key to 18b2 without any problems, so it should work - I can't answer whether the whole 18 StableRelase/Gold works with the 17 key, as long as 18 is not gold.
In any case, back up the DB beforehand (copy it into a corresponding folder) in case you have to "go back"... It's a beta, you might run into bugs...
According to some statements in the BM forums (to be taken with a bit of caution, because it's a forum) it was mentioned that with the 17 Studio you will also get the 18 Studio (Gold) (apparently so from 16 to 17 etc...)... But we'll know for sure when the 18 becomes gold, as I said ;)
LG