Bookkeeper inadvertently paid the bills from an old bank account...I wish to move to the correct bank account. Easiest way? There are more than 100 transactions

Is the old account still an active bank acct?  If not, did the bank pay them from your current/correct account, bounce them, pay them from the old account? 

However they got paid is how I would want it shown in QBs.  But give us more info and maybe someone will have a different thought.