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1. Will a player using 9a be able to net-play against a player using 9b?

No. Just like someone at 9 can't play against someone on 9a.
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Will going to 9b require converting the game data to a new format? If so, will that be reversible, or will upgrading be a one-way operation?

There won't be any data conversion. version 9, 9a, and 9b databases are compatible.
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Will custom reports (et cetera) for 9a continue to work with 9b, or will they need to be rewritten?

All your report scripts will be compatible.

Luke