There aren't any huge changes to worry about. Premium users will continue to get all the features they currently have, plus all the new features for iPhone users coming in the big 4.0 update.
There will also be a subscription tier above premium that's solely for people who want Weather Underground data on their Apple Watch. If you don't care about WU data, you don't need to worry about that subscription tier. (And if you do care about it, you can always upgrade to that subscription when it becomes available - you get refunded for the remaining time on your prior subscription, so you'd basically just be paying the difference between the two subs.)