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High mileage transmissions don't always like to be flushed since the flush does a fairly aggressive job of cleaning out the inside of the transmission and can be detrimental to the clutch packs on an older transmission. Since you have done flushes in the past you may be ok but at this mileage it is still a bit of a risk. You can always still safely do the transmission service the other way by dropping the pan and changing the filter. You don't get all the fluid out this way but you get a new filter and it is safer for a high mileage transmission.
I hope this helps. Please let me know if you have any questions about this.