If they're accepted... turning a regular Marine into a Primaris is apparently a thing, so you might as well upgrade? Not sure we've actually been told what a Primaris actually gets in extras compared to a normal.
Also note that the most willful Chapters, with the pull to resist Guilliman's desires (Space Wolves, Blood Angels) have gotten devastated to the point where rebuilding WITHOUT accepting the readily-available Primaris Marines is impossible, right before Guilliman shows up to ram the Primaris through their throats... so "Take These Or Die Out" is likely to up acceptance.
I'm guessing the regular SM kits will get phased out for Primaris-sized Tacticals/Devastators/Assault Marines over the course of this edition, and possibly the next, with GW advancing the timeline a couple hundred years, after which the Primaris will officially be the new normal.
Amusing note: Unless those are replaced as well, the SCs should still be their current size, so that's going to look funny for a while.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.