None of that - the photos uploaded at "original" quality after that date will start counting towards your Drive quota.
Same as what happens when you switch to another non-Pixel phone.
Re:Pixel 3a
by Double_a_92 on Wednesday May 08, 2019 @03:48AM
(#19856732)
What if you are above the free drive quota?
https://support.google.com/photos/answer/6220791?co=GENIE.Pl...