Você pode criar uma coluna computada persistente e indexá-la, consulte Creating Indexes on Computed Colunas
alter table add newcolumn as cast(oldcolumn as varchar(24)) persisted;
create index table_newcolumn on table (newcolumn);
alter table add newcolumn as cast(oldcolumn as varchar(24)) persisted;
create index table_newcolumn on table (newcolumn);
