You cannot import the prior year info again unless you start over from scratch and that will not correct your issue ... if anything was transferred that truly affects the 2018 return you will be able to make the corrections in the interview. 

Specifically the 2017 state tax liability amt in data file is overstated, and so 2018 program assessing an underpmt penalty based on too high 2017 liability amt. Turbotx wont let me override amt imported from 2017 data file, although I since went back & corrected the state return in the 2017 Ttx program.

In the state interview are you given the choice to let the state compute the penalty ?   And are you using the online or downloaded version ?

Using downloaded version & Turbotax calculated it - didn't give me choice. It's calculating penalty based off the 90% of 2018 liability amt instead of my corrected 2017 liability amt that's $32 lower. This is a minimal difference concerning calculating penalty. Let it ride, I guess.

This is where importing the correct prior year info is key ... You should have the option to let the state compute the penalty instead of letting the program do it.   You could also switch to the downloaded version where you can override the amount in the FORMS mode.