Variables do not have to be declared in the process archive. At runtime, you can just put any java object in the variables. If that variable was not present, it will be created. Just the same as with a plain java.util.Map.

Variables can be deleted with

ContextInstance.deleteVariable(String variableName);
ContextInstance.deleteVariable(String variableName, Token token);

Automatic changing of types is now supported. This means that it is allowed to overwrite a variable with a value of a different type. Of course, you should try to limit the number of type changes since this creates a more db communication then a plain update of a column.