Hey!
The setting isn't expected to be changed during the run, but you are correct that if the total number of packs changed, that could make some packs change recipe even though the same pack number is in both sequences. (99 total packs -> 34 should be 1st medium recipe; 66 total packs -> 23 should be the 1st medium recipe; although 23 will be in both, the recipe will be constructed differently).
Before the event began, I changed the mod so that the user could choose the first medium and first hard pack (defaulting to 25 and 50). So long as those settings and the random seed setting remained the same, the user could update from 500 to 600 packs, and the first 500 will remain the same.