Can you explain what you mean by "dismiss"? I can't figure out what that would mean apart from deleting the party on the Story Mode screen - would that delete progress (either adventures completed and/or difficulty tiers completed) when I start the new party?
No mate, doesn't work in that way. Progress is the 'sum' of your characters completed scenarios in Story mode. And you never lose characters dismissing the party. Think the party as a temporary group of characters. To remove the party tab on Story in the main screen. And you will see the list of your current active parties. There is a X button on the top of each to remove that party. You don't lose the characters, only their group. Then you can create a new party from the same screen and then select exactly your previous characters.
In general you can add or remove characters from a party without problems. Then if you remove an experienced character, the party could not to access some scenarios if he was the only character that could access them. Because every char have is own completed scenario sheet (check it to know if new chars you added miss something).
So if your progress is stuck in Story, you can remove the party (with the memory of the last scenario in progress) and simply rebuilt an other one with the same characters to start that scenario again from the beginning from the difficulty selection screen.
PS wonderful avatar
Thanks for explaining this. I was afraid I would lose progress. I started a new story and I was very happy to learn that I did not have to redo all the quests with my characters.
However, I noticed that if you swap all characters to new characters in a story, you still have access to all scenarios in any order. This suggests that you might be able to complete the hook mountain scenario to get a role early and go back and put power feats on the role card instead of just the basic powers. Haven't tried it yet though.