Well - I'll take a stab at this one, being fairly new to the SCRUM world anyway. My background has traditionally been in Big 5 (when there was more then 5) and Fortune 100 companies. So - lots of process - and lots...lots of paper. The basic axiom behind SCRUM as a methodology, is that it is *barely sufficient* (self described in training).
Meaning - there is not much holding it together, and if you don't do one of the three or four prescribed steps/processes - the whole thing falls apart. Don't let that scare you though - it's actually pretty easy.
Now getting to your question about Charter, etc. Because I'm doing the same thing... Our charter process right now is really just an initial document to help set the boundaries of a project...nothing more. My gut tells me that I need to put much more work into the charter, etc. But SCRUM says to throw it out the window and let your backlog (requirements) and sprint planning drive your delivery cycles.
In the end - another premise of SCRUM is that you are never really done. The backlog of new features, bugs, enhancements; is constantly being updated and changed. In our case, we have been working with our users to gather and define all requirements - and determined what is a must have for release 1.0. Once we know that - sprint planning/estimating allowed us to decompose the 1.0 requirements into several 30 day sprints.
After the first sprint was completed, we went back to the backlog - reprioritized with the business based on some new information, and recommitted to the second sprint. And so on...
So - as it turns out - while our Charter helped set the big picture for how the new application fits in the business with high level goals... our backlog is what really drove the effort.
Now I hope I said everything right - so if you have any questions, don't hesitate.