You're not eligible until you have paid in for a full year. You could pay it off and retail sell the phone, but not sure if that lets you get back into the upgrade program, I think you get kind of locked in? I've never tried it though.
I've either upgraded after 12 months or just kept it for the full 24 months (XS Max) and am starting fresh again on the 12 Pro Max.
EDIT: OH - unless you're planning to keep both? Then yes, it's a hard pull again... just never tried to get two phones on one IUP plan. My husband and I did IUP separately.