When I started working as the Accountant the previous Accountant had gone thru some what you are about to go thru.  She told me Lynn had walked her through the process over the phone.  I had taken over about a year after that happened.  I will say that there were a lot of errors, things not going where there where supposed to or just plain getting missed.  That was more of a lack of understanding of DBA's process and how manufacturing works from the previous Accountant.  We have a HUGE inventory list of purchase items and sales items.  All of which lead to multiple divisions for costing and income that was just one big lump of numbers.  Garbage in Garbage out!  With what I know now and I had to do what you are about to do, I would start from scratch, take my time, filter, and really visualize (a white board to draw it out helps a great deal) how things flow and what information you want to see together or seperate from each other.  Like I stated we have multiple divisions so it's important to know where things should be catoragized. I really like DBA with the exception of reports.  That's were seperating things comes in great. I still occasionally come accross something and wonder why on earth is it going there or where did that come from.  
 
You can use DE Date Exchange to export and import from old to new.  It's also a great place to filter.  By starting over with a new company, you can still keep your current information (unedited) for reference and limit who has access to it.  There have times when the bosses wanted to know information from that year when things were lumped together and I just can't give it to them without spending a large amount of time digging and filtering in excel.  Good luck!

Nothing canned because every situation is different.  POs that are hanging out as PO/RNI can be closed using PO-K and they will go away.  Open AR from Sales Invoices are trickier.  You can either go into each customer in AR-C and make lump sum payments or clear the open AR files (email me for details).  In any case, as of your "Go Live" date, use UT-K-A and clear all transactions from GL and leave all the other modules alone.  You can import open AR and AP from your existing accounting system or enter Beginning Balance vouchers manually but in any case you want to verify that open aging agrees in the old and new systems.  Make a GL Journal Entry in GL-B for the starting balances in the GL and go forward from there.  AD-A Defaults need to be set up as well as the GL mapping for inventory in SM-C.  
 
If you want some more hand-holding, upgrading to an email or phone support plan for a few months may be worthwhile.  It isn't a lifetime commitment, just a 3 month minimum.   If you are on a calendar accounting year, January 1 would be a good Go Live date and you have enough time between now and then to get all your ducks in a row.