I haven't made such a promise. In fact, the rules say that I may
change a running game:
http://www.pbm.com/oly/g2rules/000-intro.html#Game additions
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Game additions and rule changes
Features may be added to Olympia every so often, such as
new areas in the map, NPC races, magical artifacts, skills,
spells, etc. It is also sometimes necessary to make slight
alterations to existing game mechanics to correct bugs or
flaws in game balance.
While it is inevitable that some players will be affected
by rule changes, every effort is made to not disrupt existing
game positions.
This is mentioned only as a warning that the rules may
evolve over time. Changes are announced via email to all
players.
Although, as Greg points out, I am usually discouraged from changes
during a running game by the number complaints it generates. These
can often be minimized by giving a long enough advance warning of
the change.