I totally support the idea to let the user choose which Slot to use should the current run out (as long as it’s the same material).
My idea is to have all slots filled with almost empty spools and run a functional part or a prototype over night using all my leftovers. So if the user could define on runout e.g.(or any other sequence):
S1->S2, S2->S3, S3->S4, S4->S1
That would be awesome!
Keeping BL’s environment friendly attitude in mind, that should be ticking all the right boxes in that regard too 
No more waste filament…