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.
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