Use SQL to update PART of a field

Hi folks
Before I forget (like I normally do) we are on V5r4.
Within SQL, how can I update part of a field.
I think I can use the following
The field FULLFIELD in FileA is 20 characters, but I need to change the 5th character to the value A for ALL records.
The SQL would be
Update FileA set FULLFIELD = substr(FULLFIELD, 1, 4) concat 'A' concat substr(FULLFIELD, 6, 15)

However, saying that, I thought that I had read somewhere, a different method for changing the value of a portion of a field.
Obviously, I could have imagined this, but if anyone knows what I am talking about, could they put me out of my misery.
As always, any answere gratefully accepted.

Alan Shore
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