I use a 2002 Toyota Camry and of recent i noticed that even when i depress the brakes and try to shift the automatic transmission from Park to Drive, sometimes it refuses to shift and with much effort it finally shifts.My question,as it just occured to me that, i have never completely drained out the transmission fluid and replaced with a new one in 3years,could this be a problem? Also how often should the transmission fluid be drained out and replaced with a fresh one(my mechanic says as long as there is no leakage whatsoever,there will be no need to replace ever),is he correct?

Hoping to hear your proffessional opinions.

Thanks.