Yes, turn it off first. When you redeem, the system should ask you if you want to turn it on and get another month for free and then turn it off again.
FYI - When I redeemed a code a year ago, I got the option for auto pay which I turned on and got another month. I turned it off and redeemed another code right after but it didn't give the option for another free month so you may want to wait a day or so to redeem a second if you have it.
I think more likely they just caught on lol. I haven't really heard of people being able to use the trick anymore with any real frequency other than the first time, if that. MS has been exuberantly generous with their subs, but I think we're starting to see that shift towards maintaining vs attracting people.