The torque converter seal (yellow arrow) should be replaced any time you remove the torque converter from the transmission or if it starts to leak. The seal can easily tear as well as dry out over time. So reusing an old one is not recommended.
Don't forget to pre-fill the torque converter 1/2 way to keep the transmission pump from running dry for too long during the first start. Fill it just under where it will spill out when installing on the transmission and then be very careful. It can quickly turn messy.
If you ever need to remove the pump or the main pump seal is leaking, then it should be replaced also. The dark metal section in the picture is the pump. It has a few bolts around the outer edges to secure the pump. The seal follows the outer edge of the pump. My pump seal does not leak and I am not doing any maintenance/modifications inside of it. So I am leaving the old seal for now.