They will be paid out on the same schedule, but as cash (at $54.20). I assume this will also be true of promised grants with a number attached to them that have not yet been converted to a fixed number of shares yet, but that hasn't been fully clarified yet.
There will be no stock in three years if the acquisition goes through. My understanding is that effort is being made to compensate employees in a roughly equivalent manner, albeit without appreciation or deprecation being involved since Twitter will cease to be a public company.
A lot of people in public firm tech have effectively normalized not just large RSU grants but the assumption that they'll significantly rise in value. Twitter may, depending upon the agreements and Musk's wishes if this goes through, pay out unvested shares cash equivalents on their current schedule at the acquisition price. But there will presumably be no further rise and no further RSUs.
Presumably the treatment of unvested shares is somewhere in all the acquisition legal paperwork because it would need to be accounted for.
