tranny fluid and filter change

[chevy4life02814]

sorry but i have kind of an off topic question. i just got off the phone with my father. first off, some guy at the gas station told him his 96 tahoe sounded like it needed gear oil, so he tells my father, "its easy, you just put it in the crankcase". he asked me, and i told him, "don't do it". even i know that. but my father also said his transmission is slipping when he's trying to go in reverse. so i told him "sometimes" a filter and fluid change will do the trick. but i think the tranny is going, but i didn't tell him that, trying to be positive. he then tells me you shouldn't change the fluid on a car with over 100,000 miles. the tahoe has 250,000 miles and i don't think hes done a change since he got it with 140,000 miles. is there any truth that a change will screw up the tranny. i'm not talking about a total flush, just a filter and fluid change. thanks, sorry if i rambled on...
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2011 camaro
1987 r10 p/u
1989 scottsdale stepside