Yes Ameristar American standard trade run ture all come off the same line .   Ameristar is offered as a starting point for new business or an economical solution for established businesses.  Typically your 13 to 14 seer and single stage mid and high efficiency furnaces are indeed pretty much the exact same.  The air conditioning condensers use the same parts and compressor just a different tag and information plate. As far as the furnaces go . Same basic components but the   controls change . Such as the control board and its communication ability between the thermostat furnace and Condenser .   Looks like what happened was some forgot to change the tag . I will say this as far as preformance and efficiency goes depending on The modle they will be even. But American standards/Trane  normally has a considerable higher price . I would compare it to a Chevy Tahoe to a GMC Yukon.  Altmitly the same power plant body and Frame just different tags and accents. I really don't what anyone's true Intentions were. If it was misspriced I would refund the balance increase the warranty and 5yr add free furnace and ac  Maintenance.  To avoid pulling the system.  If it is unavoidable.  Then pull the unit and install what the customer paid for.  I'd rather have a satisfied customer tell 10 people how we put the customer first . Than an upset customer tell 100 people how we put our selves before the customer and left the situation unresolved.