If a customer uses 4000 Sales for their income account for an item, and later down the line changes it to 4300 Sales Services instead, what is the repercussions of that change? • Will it change on open transactions? • Will it change old/closed transactions? • Will it change in transactions where the period has been closed?

From memory, it changes all of the above - all historical transactions are changed. NetSuite includes this note in the help section:

Important: Be careful when selecting accounts. Problems can occur when you try to change an account on an item record after an item transaction has been entered. Contact your NetSuite account representative to update an account on an item.