No, you cannot change the amount of the bounty.
Moderators can revoke a bounty. They might be willing to do that if you pledge to put a higher bounty. However doing this after 3 days is somewhat unfair as the new bounty would be active for a week, not the remaining 4 days. Moderators could also decide not to take the risk that you won't set the bounty again, or decide that it's too much hassle.
Putting a higher bounty hardly attracts more attention anyway. Leave the 50 rep bounty for a week, and if nobody answers, put a 100 bounty for another week. You'll get more attention this way. If somebody answers during the first week and you really like the answer, you can reward that answer with another bounty after the fact.
Also consider that maybe nobody on the site has read this story, in which case all your bounties are in vain. Try to advertise or ask in other places, and report back if you found the answer. For example, you could post a YASID on the Usenet group rec.arts.sf.written.