2004 Hyundai Elantra trans problem???

I have a 2004 Hyundai Elantra, the car only has 125k miles. The car starts up just find and idles perfect. When driving the car it has a hard shift from second to third gear and again from third to forth. From first to second seems to have no problem. Have already changed the fluid and did not seem to work. Any one have any ideas? Been reading it could be the transmission solenoids but the they are inside the pan and seems like a lot of work to get the cover off. Anyone know of anything else it could be?? Also no check engine light on at this time.

Also I changed the input and output speed sensors. But got those used so not sure if they could be the issue but don't see all th used sensors being bad.

I had this same exactly problem with my Elantra 2009 and as you said… it was the solenoids. Bought them on eBay and took me like 3 hours to do it. It was my first time and everything went well. I also changed the filter and put new fluid. This solved my problem. Now it shifts smoothly.