"I would like to go back several years and reclassify field techs into a separate COGS payroll account."
Have you looked at the Tax Filings, to see how this was reported?
"would I have to go into each paycheck and reassign the account (if that is that even possible)?"
The Payroll items control the flow of the data to the accounts. First, make sure the historic changes are not going to result in a change to tax filings, requiring amended reporting. Don't make historic changes so casually.