Hi, definitely later than Apr, because the One UI 6.1 update will be initially rolled out to these 9 models first. So, the older models, including last year's mid-range will most likely be in the next batch, which could be in May or June.
But of cos, let's hope that after the One UI 6.1 update has been released for the first batch of 9 models, Samsung has alr been working on the older models, ready to release the public version in weeks.