Why won't transmission shift into 3rd when it is cold but will when warm on 2002 Toyota Tacoma

This happens whenever the vehicle has been sitting for more than an hour

Asked by for the 2002 Toyota Tacoma
May have a sticking valve in the transmission valve body, have a transmission shop check it out/service the transmission.
The guy at the shop said the seals are hardening and the only fix is a rebuild. Thanks for the help.