It appears the only way to do this is to hack the tacacs service definitions in the /etc/services file from expert mode; I just tried changing the TACACS port to 149, rebooted and it worked. The /etc/services file is not auto-generated upon Gaia system startup so your changes should stick.
However be sure to document this /etc/services file change as it is likely to get overwritten by a version upgrade or even possibly a Jumbo HFA installation. You'll need to manually check that your port change survived after either of these types of operations.
Attend my online "Be your Own TAC: Part Deux" CheckMates event
March 27th with sessions for both the EMEA and Americas time zones