Updating statistics, what about existing execution plans If you update the statistics on tables that have never been updated (don't ask why they never have), does SQL Server automatically refresh the execution plans based on the outdated statistics? If not, what is best approach to do so?
If you update statistics on a table that didn't have any modifications since the previous statistics update, your execution plans will not be invalidated.

If you update statistics AND data has been changed between this and previous statistics update then it will be invalidated if the database has the AUTO_UPDATE_STATISTICS database option set to ON. Note that affected plans are not immediately recompiled; each plan will recompile when it is next retrieved from cache for execution.

See this post by Kendra Little: Does Updating Statistics Cause a Recompile if No Data Has Changed?


  Finding: Statistics Update Alone Didn’t Cause a Recompile
  
  SQL Server was smart enough to check if the data had changed. Updating
  statistics alone doesn’t always invalidate execution plans.


And this post by Kimberly Tripp referencing a no longer available connect item: 


  It’s NOT a bug, it’s BY DESIGN. And, it actually makes sense.
  
  If the plan should NOT be invalidated (directly due to statistics
  because the data has NOT changed) then it won’t. But… If the plan
  should be evaluated (statistics have been updated AND data changed)
  then it will.


Also see this post by Erin Stellato: Statistics and Recompilations which references this MSDN documentation page


  When the AUTO_UPDATE_STATISTICS database option is set to ON, queries
  are recompiled when they target tables or indexed views whose
  statistics have been updated or whose cardinalities have changed
  significantly since the last execution. This behavior applies to
  standard user-defined tables, temporary tables, and the inserted and
  deleted tables created by DML triggers. If query performance is
  affected by excessive recompilations, consider changing this setting
  to OFF. When the AUTO_UPDATE_STATISTICS database option is set to OFF,
  no recompilations occur based on statistics or cardinality changes,
  with the exception of the inserted and deleted tables that are created
  by DML INSTEAD OF triggers. Because these tables are created in
  tempdb, the recompilation of queries that access them depends on the
  setting of AUTO_UPDATE_STATISTICS in tempdb. Note that in SQL Server
  2000, queries continue to recompile based on cardinality changes to
  the DML trigger inserted and deleted tables, even when this setting is
  OFF.


So if you want to be absolutely sure your plans for those tables are recompiled you will have to recompile them your self using 

sp_recompile 'tablename'