Thanks for reaching out. To clarify, only when your channel is monetized, you'll have an option to select the videos which you would like to change the monetization settings on. Hope this helps.
just lies about monetization, look here 2 channels ho dont have original content and they have only reused content but they are monetized https://www.youtube.com/channel/UCiDRA0Fl2btJHP3nX-CpGcw …https://www.youtube.com/channel/UCdogB4_ht9f_oqolYleSIGg …