Well any old card that could target players would get errata to target planeswalkers. People just need to learn which sets are before 2018.
It's not that simple. For cards like
Lightning Bolt, just adding "or planeswalker" would be enough, but there are a lot of edge cases to consider. Some cards have additional effects, like
Blightning, so they could either make that part do nothing if it's targeting a planeswalker or make it affect the planesawlker's controller. We don't know which, yet. Then there are things that require information about the targeted player, like
Runeflare Trap. Maybe these won't be able to target planeswalkers at all, or maybe they'll use information about the planeswalker's controller. (And, of course, anything that doesn't involve damage at all still wouldn't be able to target planeswalkers.)
Then there are the various non-targeted source of damage. Spells and abilities could deal damage to each player/opponent, or to the defending player, or to the controller of something, or to the player that caused an ability to trigger. Right now, any of those can be redirected to a planeswalker. My guess is that they'll just lose that functionality, but they could surprise me and issue some sort of errata to some of those.
On the other hand, I don't think players will need to know when a card was printed. If they do make a sweeping change like "all burn that could target players can also target planeswalkers", then they'll also apply that moving forward. In other words, in that situation, if it says it can target planeswalkers, it can, and if it doesn't say it can target planeswalkers (but can target players), it still can, no matter when the card was printed.