Honestly nothing will really change that much. Whether something is NQ or HQ is independent of the size of the fish so instead of using the percentages to decide whether a fish is NQ or HQ, Mooch will likely be usable when a fish is in a certain upper percentile of its size range. Actions like Patience and Patience II will raise the floor of the size range so that the likelihood of being in the Mooch range is high, which is pretty much the same function as before.

The same size range could be applied to Ocean Fishing too, where sizes that fall into the Mooch tier are given double points just like HQ fish are now.

As for the achievements, they said these would be adjusted to work with the new system so you don't have to worry about them being removed.