2000 Hyundai Elantra Tran Fluid Change?

Hey, got oil change, some other stuff done today and while there, they checked my trans fluid and showed it to me. This car is a 2000 Hyundai Elantra GLS with about 135,000 miles. Don't know the last time trans fluid has been changed, but it was pretty burnt. Now, big question: I have had flushes done on trannies before and they've fried the trannie in two different cases. But these guys aren't going to flush it just do a "oil change for the transmission" They say it won't ruin my high mileage vehicle because the old stuff doesn't get flushed out, just emptyed and refilled with new, so the old buildup should protect from the harshness of the new fluid. Is this accurate? I DO NOT want to ruin another trannie ever again LOL And I really love this little car. Need some advice. OH YEAH: It does have a slight hesitation going from 1st to 2nd. Hence me really wanting to get the fluid changed. But not if it's going to make it worse.