Warm weather transmission problem

My 98 chevy astro van has a hard shift between first and second in warm weather only and that is only after driving for an hour. Seems to shift on time but there is a clunk when it does. No problems with any of the other gears. does not happen in cold weather. took it to a transmission place and the guy said he would check it out but after doing a regular service but he only drove it for 15 minutes and said it drove fine. First time I drove it more than an hour it did it again. Help

Is it an automatic transmission? How many miles since your last transmission fluid change? Has it had regular transmission fluid (and filter) changes since it was new?

Yes automatic transmission. Was purchased used about 7 years ago but I have only been driving it for the last few years so I dont know what transmission maintenance has been done on it. It belongs to a small non profit but only two people have driven it. This problem started late last summer. When I took it to be service the guy said it smelled bad.

Have the fluid changed and hope it isn’t already damaged.

did have it changed last month. If damaged is it repairable or will it have to be replaced? Are there parts that can be replaced? and why only in warm weather