How to convert in OpenERP from one chart of account to another?
IMO its very difficult we are currently migrating some data and its proving to be difficult.
I would advice you to pick a date in the future and tell everyone to just use another db with the correct chart of accounts.
Your finance dept will be the one to suggest what date  is perfect. How about when a period starts.
I have installed chart of accounts A for company1. This chart was used couple months for accounting. How can I convert into chart of accounts B and keep old data for accounts (debit, credit, etc.)? In other words, is possible migrate data from one chart of accounts to another? Solution could be programmatically or trough web client interface (not important). Virtual charts of accounts can't be used. Chart of accounts B must became main chart with old data. 
Every advice will help me a lot. Thanks