How can I change the salary journal of pay runs processed in the past, to the new GL accounts?
You will have to reopen these pay runs in Process/Progress & History. The reopen button becomes visible when you hoover your mouse above the pay run you want to reopen (see below).
Reopening pay runs of the past might however change the net salaries, premiums and taxes. This might happen for example when taxes and premiums have changed in the meantime. So be careful!