Hello

Pl see attached technical specs and wiring of the two modules.

The main difference between 6ES5 451-7LA11 / 7LA12   and  6ES7 451-7LA21 is that 7LA21 module has additional short circuit indications. The Pin configuration is also same except that -7LA21 has some additional pins assigned.

Therefore generally you should be able to replace the 7LA11/12 with 7LA21 .
However the opposite is not always true.

Just ensure that the extra pins assigned in 7LA21 are not connected.

Hope this helps

Cheers

Navnag .