There are a variety of ways to use the value in field 1 to determine what text appears in field 2 that do not require a script:
If ( Left ( Field1 = "J" ; "I" ; self )
Could be used as an auto-enter calculation for field 2.
You could also set up a related table of records and set up field 2 with a looked up value field option to copy a letter into field 2.
And if field 2 does not need to be editable you can make it a calculation field with:
If ( Left ( Field1 = "J" ; "I" )
Ps. and when you use a script to put data into a field, Set Field is a better option than Insert Text in almost all cases.
I don't think I did a very good job of explaining myself. If field 1 has a value of J0069 or J0076 or J0100 I want to replace the contents of field one with the letter I. So field 1 would go from J0069 (or whatever) to I. Is this possible?
Set up an auto-enter calculation on Field 1.
If ( Left ( self ; 1 ) = "J" ; "I" ; self )
Clear the do not replace existing value check box.