The military in WWII had different classifications for issue of items of all kinds
1 standard, the current standard issue item which supersedes any previous similar items and is the priority for production and issue
2 limited standard, a classification for items that are still approved for issue but are no longer the preferred or standardized issue item
3 obsolete, items which no longer met the basic requirements for issue and are subject to being withdrawn from service
So when the B15 became standard the B10 and A2 would have been moved to the Limited standard category, they weren't obsoleted during the war
There's also a sizeable delay between standardization and front-line issue. I searched thousands of photos online and in books for a few years for the first sight of a B-15 in the ETO and found a few appearing on bomber crew photos etc. dated December 1944. That's the earliest I found. Also the B-10, standardized in July 1943, first seen in ETO pilot/crew photos in April 1944.