Well transferred points maintain their original Use Year (so October 2021 stay October 2021 even if you are September, etc) when transferred, which is important when you are to consider when transferred points can be banked, which is currently up to their Expiration date (at least that is what I was told for points I've received) but you might want to verify this point.
So say you have December Use Year and they have February you would have quite an overlapped period where your 2020 points are valid while their 2021 points are valid.