Just a stupid question - if your firmware is on 1.7 can you skip 1.8 and go directly to 1.9? 
Most likely yes.
In any case you should always check the release notes because sometimes updating from an older version is not possible directly. If I remember correctly with DNS-323 it wasn't possible to update from 1.03 straight to 1.08. You had to update to 1.05 first and then to 1.08. But that's usually the case only when you're updating over many versions at once.
Hope you get what I meant.
