All ATF's have foam inhibitors so I ain't buying the foaming from mixing.

The fluid change danger in high-mile transmissions is an ongoing debate and, again, anecdotal experience varies. Some believe that a fresh additive package will dislodge varnish buildup and cause problems while some believe that the abrasive nature of aged fluid prevents clutch/band slippage and fresh fluid, being slipperier, will cause slippage.