Feherke,
Let me start by saying thanks. Your corrections & advice are helping me understand this a little better, and I appreciate that.
I have modified the code:
awk -F, '!sub(/=FOLK:SHPD=/,", ",$20){$20="\", \""}1' test_input.txt | awk '{OFS="," ; print $0}' > test_output.txt
I'm trying...