While I've only heat molded my own boots, I personally doubt that heat molding them will make them tighter at the balls. The real question is whether they will make them less tight.
But you went from 7.5C to 7.5B - which means, if Jackson follows the regular pattern that I found for shoes online, that your boots will be 1/4" less wide. You may have a very tight fit width-wise, at the ball, if your old ones were already pretty snug there.
Also, each pair of boots, even if they are sized the same, is likely to be a little different, in most brands. I don't know the reasons for that.
I assume you mean at the ball, but right at the bottom of your foot, not higher up right? Higher up is easy, and most heat moldable boots can handle it. But right next to the sole, may not be moldable.
Jackson's video
https://jacksonultima.com/pages/boots-products#2000SeriesBootsdoes not say exactly where the heat molding material is, so it is hard to guess whether it includes the ball at the bottom.
I think that video implies spot punching - where the fitter uses a machine to physically stretch out the material in spots - might cover that area - but maybe not - again it is hard to guess if you are talking about the area closest to the sole of the boot, which might not be stretchable. I have personally punched my own boots around my toes and ankles this way - but the area punched wasn't immediately next the lowest part of the boot, which is of course harder to stretch, because the outsole and midsole don't stretch, on any boots that I know of.
If it is still too snug there, you can try to make yourself a custom insole that is thinner than the one that comes with the boot, especially at the ball. That might or might not work. In particular, it places your foot lower down in the boot - which might be thinner or wider. You could also try adding a little extra thickness, or just placing a thin dollar store insole (or for temporary test purposes, a thin piece of cardboard cut to the shape of the bottom of your foot) on top of the old insole. In most cases that makes for a tighter fit - but if the only part that is too snug is right at the bottom of your foot, and there is room above your foot, it is barely conceivable it might help. I've never heard of anyone trying that, but in theory it might help.
It also is just barely possible that a thin rigid (probably heat moldable) insole or orthotic could push outward on the fabric of the boot there, so that your foot doesn't have to press it outwards. I did once have such insoles make one of my pairs of boots too large for me in exactly that fashion.
But until you get your boots and try them, it is really hard to guess anything.
This type of problem is the reason a lot of people are willing to drive 100 to a few hundred miles to a store where they can actually try out their boots before buying them, and why they don't mail order. It's outrageously expensive and time consuming, but some of us have had no end of foot problems with boots that didn't fit.
I hope yours do fit!
P.S. If you have been skating most of your life, but shortly after taking lessons, they started to break down, there are a few obvious possible reasons:
1. Maybe you are skating more.
2. Maybe you are doing more difficult things. Jumping, and deeper edges, both put a lot of stress on boots. (I'm not suggesting you don't do those things.)
3. Maybe the pain of the tight fit meant that you laced less tightly - which often makes skates break down very quickly. I used to work at a skate counter, and people sometimes completely broke down rental skates in one session, because they didn't lace tight enough.
4. Maybe they were simply getting ready to break down.