I will post on this topic based on my personal experience (and other cases I've heard of personally). I'm non-EU as well and had a L for the first 2 years of my employment; got a B thereafter. I don't work for one of the big banks/companies in Zurich.

AFAIK, there is nothing like a specific quota for a given company. Kantons have a sharing system for the permits but not the companies. Again it could be possible to do so for the big 3 banks or another big MNC, but I've not heard anything like that at least until now. Even in that case, it should have been 3-4 months ago and not now.

I can certainly tell that there is nothing like a special form (or request) for L to B conversion. Your employer should submit all the documents to the AWA emphasising why a Swiss or EU person can't fill your position and then it's upto AWA to decide on a case-by-case basis.