Until, recently bank statements were added manually, but we decided to connect clients account online, instead connected existing account, new one was created resulting in 3 months of duplicated transactions now. I know its possible to merge accounts but how to do it correctly so i keep the new live bank account and all the transactions from the old one that are already matched? 

Old account is is done up to a march, new one starts from January. And Does the old one has to be reconciled first ?