HI
I suggest below approach
Ask them how they want to manage the financial data in the old system + Business central system or just the business central system
If they say we want to close everything we have done till the end of month in Old system and starting 1st June we would be using Business central
So then you would be carrying out below set of actions
1. Open balances for customer, Vendor and GL , Bank etc would be moved to the new BC system you could either have this posted for each invoice per customer or just a total amount which you could pass via GL entry
2. Ask them to close all sales, Purchase order in the previous system and whatever amount is left recreate those in the BC system example an order was placed for 20 Qty and 10 was shipped, Close that order in legacy system and create a new one with 10 Qty in BC system
3. For the change of COA you do need to know the old and new mapping and then apply those in the new system
Ideally the best way is to do a parallel entry in both systems till the month end or something and then move over to BC on a weekend which you decide as the cut off date
Hope that helps
Regards
Vinita
