How to convert in OpenERP from one chart of account to another?
The fastest way to do so is using a ETL like Talend or Pentaho (provided there is a logic as to which account will map to which other during the process). If not you will have to do so by hand.
In case there is a logic, you would export it to a format you can transform and re import. Uninstall your account chart and install the new. Then import all the data that you formatted using those tools.
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