Adding perf groups to packages?

I haven't run across anything here or in documentation that says otherwise, but I think I know what the answer is...

 

We want to adjust our package definitions (flex packages are the only ones that would get the adjustment) to exclude the first show of the season, and add a new option that we're announcing next week. Judging from my inability to click on Add and Delete in Ticketing Setup > Packages and have them perform said functions, my guess is that existing packages that have sales in them already cannot be changed (which makes sense after a fashion), but I wanted to confirm the hunch or have someone leap to the rescue with some fancy Tess ninja-ing.

 

Thanks to all.