SQLAnywhere: Rename Foreign Key or Primary Key
You don't say which version of SQL Anywhere you are using, but in 16 I think you can use ALTER INDEX to do what you want. From the help:


  ALTER INDEX statement
  
  Renames an index, primary key, or foreign key, or changes the clustered nature of an index.
  
  Syntax
  
  ALTER { INDEX index-name
   | [ INDEX ] FOREIGN KEY role-name
  | [ INDEX ] PRIMARY KEY } 
  ON [ owner.]object-name { REBUILD | rename-clause | cluster-clause }
  
  object-name : table-name | materialized-view-name
  rename-clause : RENAME { AS | TO } new-index-name
  cluster-clause : CLUSTERED | NONCLUSTERED  
  
  Parameters
  
  rename-clause   Specify the new name for the index, primary key, or foreign key. 
  
  When you rename the underlying index for a foreign or primary key, the corresponding RI constraint name for the index is not changed. However, the foreign key role name, if applicable, is the same as the index name and is changed. Use the ALTER TABLE statement to rename the RI constraint name, if necessary. 
  
  cluster-clause   Specify whether the index should be changed to CLUSTERED or NONCLUSTERED. Only one index on a table can be clustered. 
  
  REBUILD clause   Use this clause to rebuild an index, instead of dropping and recreating it. 
  
  Remarks
  
  The ALTER INDEX statement carries out two tasks:
  
  It can be used to rename an index, primary key, or foreign key.
  
  It can be used to change an index type from nonclustered to clustered, or vice versa.
  
  The ALTER INDEX statement can be used to change the clustering specification of the index, but does not reorganize the data. As well, only one index per table or materialized view can be clustered. 
  
  ALTER INDEX cannot be used to change an index on a local temporary table. An attempt to do so results in an Index not found error. 
  
  This statement cannot be executed when there are cursors opened with the WITH HOLD clause that use either statement or transaction snapshots. 


Note what it says about using the ALTER TABLE statement to rename the constraint name if needed. If you need more help try the dedicated SQL Anywhere forum at http://sqlanywhere-forum.sap.com/