Not quite, the total you roll on those die is your new HP, if you happen to roll under your current HP, instead of losing HP, you just gain 1. This makes it so characters with low HP are more likely to gain HP than those who already have lots still get something.
Ah, I am just not parsing the language in the book the same as you meant it! "When a PC gains a level, they roll a number of d8s equal to their new level to find their new HP maximum." My brain processed that as "Roll d8s equal to their new level and then consult what we say next ("find") to determine the new HP maximum" and that's the part about rolling under. The "find their new HP maximum" is what threw me, as it suggested I was looking something up, which in this case, is if the number is lower than my current HP score or not! You might want to consider making that a little clearer. I might suggest "When a PC gains a level, they roll a number of d8s equal to their new level. Compare that to their current HP maximum. If it is higher, this result is their new HP maximum. If it is the same or lower, increase HP maximum by 1." That has a very clear if/then A or B argument, rather than -implying- A but stating B explicitly. Thanks!