MERGE and the Cassette Buffer File/Tape Buffer
Many or most versions of BASIC on these machines would have had a "Cassette Buffer File/Tape Buffer", and many would also have had a command like MERGE .
. . .So maybe the commands OPEN "CAS: PROGRAM" FOR OUTPUT AS #1 and PRINT #1,"10 PRINT 1"or one of the Save to Cassette commands could write a program line ( or short program ) to that "Cassette Buffer File/Tape Buffer", and then using the MERGE command to put( or replace ) a line in the main current program with what is in the tape buffer.
. . . NOTE - Obviously I mean using the tape buffer without even needing to plug in a tape-machine .
( Interestingly, some programs for the c-64 were stored on and ran from the "Cassette Buffer File/Tape Buffer" . )
AUTO
GW-BASIC and others had a command like AUTO which generates new line numbers ( so you don't have to type them ), but I don't know if it could be used from a program.
. .If AUTO would function from a program, then maybe lines could be replaced or added ( using AUTO and the PRINT command to enter text and ENTER )
NOTE - I posted another question at the same time as this one asking how many programs could be on these machines at the same time / concurrently, and one of the answers would have been identical to this answer I have posted ( although at the time I either just wasn't fully believing that tape-buffers etc existed, or wondered if it worked another way ). Unfortunately the other question I posted has been totally deleted of this site, not by me, I would be happy to delete my answer, maybe an issue for admin