Interesting question. But I don't believe it does change the carb content.
If anything, toasting would probably caremelise any starch on the surface of the slice?
On a general level, an NHS dietician told me that you can calculate a home made meal by just adding the constituents. That would suggest that cooking makes little difference. I've been using that assumption for years with no issues as far as I can tell.
A quick google throws up all manor of "benefits" with toasting. Including lowering the fat content??
They apparently have also checked the effects of toast meassuring BG levels. I'm assuming the subjects were all running normal metabolisms.
From my experience a slab of starchy food is still a slab of starchy food even if frozen and then scorched hot - maybe I don't know the right incantation, but the magic just doesn't work for me.
This sounds like a reasonable idea.
Start with the "foundations" carbwise per slice (in this case lower carb bread.) on what you are planning to toast.
Then armed with a meter. Check it's effects on Blood glucose..