I think it's possible to "drop" your bounty... and not decrease it
Meaning.... someone like X-Drake is working for Marines "secretly" but has a bounty on his head now
It's possible I believe that if him working for Marines GOT OUT and his mission is complete/accomplished.... it's possible that WG can issue an order of dropping his bounty... and canceling it...
On the other hand, bounties do not decrease in my opinion because we have seen old geezers who remained with same bounty... as an example, Don Chinjao remained with that 500+ bounty...
There might be an exception though
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If his true identity as Stealth Black stayes hidden, I'm afraid he may get a seperate bounty to his Vinsmoke one. He may become the first strawhat and character that manage to possess two seperate bounties for his head for his two personas.
Doubt it cause WG already know now that's he's called Vinsmoke Sanji
if so, I believe they can connect the dots that the only Germa guy who is always with SHs is Sanji... thus stealth black should be him...
In fact, if Oda was going to pull something like that off... he would have done it with "Sogeking" and "Usopp"