What would you need that for? Any implementation of global blocks would be there to merely help live performances. Sure, there's some other thinkable scenarios, but global blocks very likely wouldn't make much sense there.
Oh, I thought that was obvious: to save precious time when editing a bunch of presets.
Imagine I have 27 presets in a setlist, and one day, at home or the rehearsal place (not live), I try a different mic in a cab, and I realize I like it more than the one I had. With a global block, once I'm happy and I decide I want that for "my sound", I can easily spread the change to the other 26 presets just by saving. Otherwise, I would have to go over the 26 presets to repeat the operation. It's like having a variable in programming.
Again: What if you tweaked your analog amp for an hour? How to get back to the pre-tweaked settings?
And if you really spend an hour tweaking things, that's most certainly nothing one should ever use a globalized block for. So you'd de-globalize it and tweak and then possibly globalize it again. That's UX 101.
Apart from that, there's an undo function.
Hey, you stole the "that's UX 101" from me. Not fair!

The good thing of digital is that you can do things you couldn't in the analog world. For instance, in current Helix, you could be tweaking your amp block for an hour and then cancel the update if you didn't like it.
Your approach of de-globalize and then globalize again would work, sure, but would be miles away in QoL to just tweaking the global block and then save it once you're happy. It saves you many clicks.
On the other hand, have you ever used Undo to undo dozens of actions? Not the coolest thing to do.
But I understand you need immediacy for your use case, so a flag in Settings for that would make sense.