Can I modify these to help get the high note?
Absolutely! One of the biggest mistakes I think mouth call users make (I know I did for years) is to scrap calls that they don't like. They just assume it is not going to work for them. Very minor changes in the cut configuration of a call can make big differences in how they sound.
For the high/low rollover on the yelp, the fix may be as simple as exposing more of the secondary reed by trimming the top reed a bit. Regardless, there are a number of things that can be done to "tune" mouth calls.
Just remember to make those modifications in very small increments and try the call between each. However, another solution is to make larger modifications by just changing the overall call design. By this I mean, for example, changing a V-cut into a combo cut,...or a batwing,...or a ghost cut,... by removing tabs/wings.
I have found that it is the rare mouth call that does not have a "turkey" in it somewhere. You just have to find it. Bottom line is that if you have a call that you don't like and know you will not use, rather than throwing it away, why not mess around with that call to see what happens to the sound if you change it a bit?...