There's at least one reason for which it would be unsafe : Wiki pages custom permissions won't work for most pages in 1.8.3 anymore, those will fall back to global permissions.
There are good chances 1.9 won't even work due to DB structure changes. What happens when a query expects more columns in a table?
If you just want to experiment 1.9 with that DB, make a backup and upgrade it to 1.9 and run your tests. Although I never heard feedback from someone trying to do what you say, I don't think it would be positive.