Not to give you false hope, if long time no change AT gear oil you might in surprise after change new AT oil. Due to long term of oil degration & wear & tear, the gearbox internal already suit with the dirty, thicken up AT oil. There will be possibility of even worse gearbox feel after change of fresh AT oil. So be prepare, 50:50 chance of getting it as i already seen a lot of case at my mechanic shop due to neglected oil change. Mazda skyactive a lot of owners 50~60k all go out change the AT oil themself and already feedback very dark, dirty AT oil.

Easy said, if your AT oil did not change frequently like every 50~60k your internal gearbox mostly already wear a lot.
Since you mentioned 260k on meter now, if long time no change like over 100k no touch the gearbox oil until now, you might got 50% chance of gearbox got issue after fresh AT oil change, as the new AT oil will make your gearbox felt even more obvious of the wear & tear if long time neglect.

Actually my mechanic also tot about it, usually for these old neglected gearbox they will advice owner to open up the AT oil sump below and clean and change the AT filter thus so call "deposits" mostly is not the main reason. After the AT oil change they will do AT gearbox calibration again, usually after calibration done no issue than owner save a lot of money.

Seen few case after do but worse condition 1 when open the AT gearbox for checking they found the gear clutch pads mostly already worn, previously due to oil is thick and dirty the oil itself help stick the clutch pads together during shifts to prevent slips. The clutch pads are similar to small brake pads which the function is to prevent slips when engaging gears. (AT gearbox inside got many layer of this small clutch pads and steel plates for each different gears.)
So when new fresh AT oil go in, due to the clutch pads already worn it make it more easier to slip.

Actually AT flushing just make it worse, as the debris might dislodge and stuck in important areas like bodyvalve if force flush. Previously one of my favorite mechanic also got 1 AT flush machine, after few car use and encounter the weakness already ditch the machine at the side.
Like some member feedback, the safetest way maybe change 50% of AT oil first, shorten the oil change intervals until 100%. I believe that will be the lowest risk.