I tried once more with the following changes.
Same library for SYSUT1, and SYSUT2, both with disp=OLD.
...and got the same result.
I made sure the library and member are there.
...and the member is in the correct sequence with increments of 10 for line...
This is the job I used.
//JS010 EXEC PGM=IEBUPDTE,PARM=MOD
//SYSPRINT DD SYSOUT=*
//SYSUT1 DD DISP=OLD,DSN=BX220.FROM.JCL
//SYSUT2 DD DISP=OLD,DSN=BX220.FROM.JCL
//SYSIN DD *
./ CHANGE NAME=BX220RTC,LIST=ALL
./ NUMBER SEQ1=070,NEW1=073,INCR=05,INSERT=YES
//* THIS IS THE FIRST COMMENT LINE I WOULD LIKE TO INSERT IN THE *
//* AND THIS IS THE SECOND LINE I WOULD LIKE TO IN INSERT IN FILE*
...and this is the result.
SYSIN NEW MASTER
./ CHANGE NAME=BX220ARTC,LIST=ALL 0
./ NUMBER SEQ1=070,NEW1=073,INCR=05,INSERT=YES 0
IEB807I INVALID OPERATION.
IEB818I HIGHEST CONDITION CODE WAS 00000004
IEB819I END OF JOB IEBUPDTE.
Something doesn't like it.... but what is it?