AO Classic and upgrade to online tax

When I upgrade to the new online AO, does the efiling functionality still work in AO classic until its turned off in April 2021. Meaning if I have upgraded to the new AO online and I need to efile a 2018 tax return, can it still be efiled.

Also if I have unfiled 2019 and 2020 tax returns in classic that are waiting to be efiled will they come across into the new online tax so they can be filed from there. If, because they are unfiled they don't move into the online platform, can they still be efiled from AO classic as long as its before the 16th April.