All these questions can only push the rules we know to the limits, since this case is not covered anywhere... So we enter the land of reasonable speculation...
I found this thread on BGG discussing a similar scenario back in 09/2012:
https://boardgamegeek.com/thread/850355/flier-castles...so you can see we continue on the land of unconfirmed rules in order to prevent the game from breaking.
In general terms:
* Players without the majority would be removed their meeples from the small town without scoring (the same as in a normal city).
* Then, each player sharing the majority should decide in turn if he/she scores the small city or converts it into a castle.
- The players sharing the majority and scoring the city would score their points and remove their meeples from the small city.
- The players converting the small city into a castle would play a castle token each on the small city and place their meeples in the castle on top.
- You may place only one castle token and remove the rest from the game.
Note that each players needs a castle token. Players may not reuse some others castle token. It would be unfair. This means that players who ran out of castle tokens can only score the city if sharing the majority.
[237.c] What happens if the blue meeple is a big meeple?
Let's check a couple of scenarios...
Scenario A (the one you requested):
1.
places a semicircular tile with a regular meeple.
2.
places a flying machine tile and lands a large meeple on the semicircular city.
3.
completes the small city.
would like to convert it into a castle but
would like to score it.
In this case
has the majority so,
cannot do or decide anything.
would score 4 points for the small city and both meeples would be returned to their owners.
Scenario B (a variant swapping players' wishes):
1.
places a semicircular tile with a regular meeple.
2.
places a flying machine tile and lands a large meeple on the semicircular city.
3.
completes the small city.
would like to score it but
would like to convert it into a castle it.
In this case
has the majority so,
cannot do or decide anything.
would convert the small city into a castle and place its meeple in it.
, in minority, would remove their meeple.
[237.d] What happens if both meeples are normal meeples, but is moved from the City of Carcassonne the big black meeple (into the castle)?
Let's see the scenario...
Scenario:
1.
places a semicircular tile with a regular meeple.
2.
places a flying machine tile and lands a regular meeple on the semicircular city.
3.
completes the small city.
would like to convert it into a castle but
would like to score it. Since the city is going to be scored,
decides to deploy his/her large meeple from the city of Carcassonne.
Since
has the majority, the other players cannot decide anything and the small city has to be scored (
cannot do any other thing - the deployment is aimed at scoring). So
scores 4 points and all the meeples are retuned to their owners.
[237.e] What happens if both players wants to convert it into a castle?
Let's see the scenario...
Scenario:
1.
places a semicircular tile with a regular meeple.
2.
places a flying machine tile and lands a regular meeple on the semicircular city.
3.
completes the small city.
and
would like to convert it into a castle.
Both
and
need a castle token each. They both place one castle tokens on the small city and place their meeples in the castle. (You place one castle token and remove the other from the game).
Both meeples would score the full points for the next completed feature (in a subsequent round of scoring).
You may have additional scenarios if involving mayors, the fairy (migration to a castle? = undefined
), ringmasters (scoring ringmaster points in a castle? = undefined
),...