TL;DR
Blueprints mark for upgrade if existing entity is upgradable to entity in blueprint at that location.What ?
Placing a blueprint of an entity exactly over an entity that can be upgraded to the entity in the blueprint marks the entity for upgrade.Or, in pictures:
Placing this blueprint over this base results in this (Apologies for the half-length smelter and the creative mode power source. Imagine they're full-length and there's ten of them next to each other)
Yes, this won't work cleanly with braided belts, but neither do upgrade planners in general, so it's not introducing a new bug.
Why ?
Because upgrade planners in multi-stage blueprints can be really fiddly, and its too easy to miss that one random important thing.Any fiddly build where you have to upgrade some of it but not all of it will benefit from this. Malls and smelters are the obvious use cases, but there are others.