If you try to get a bonus on Premier or Prestige before 24 months, Citi is likely to deny you because of the "open" of your Rewards+ card. That's not based on anything Citi says (Citi agents don't always give correct or complete information anyway), it's based on datapoints; if you get a card in the same family as a new card number, it has always counted an "open" in reference to 24-month "opened or closed" language before, and I see no reason to expect that it wouldn't in this case. (And, btw, if you try to apply 2 months too early, and you're denied because of that, you've just reset your 24-month clock again. So "hoping'" that you're not under a 24 month clock can set you back even way more than "patiently waiting" for a "no doubt about it" time.)

Can I ask why you want to convert to the Rewards+ card, instead of the AT&T Access card, which is also a no-annual-fee card, also earns Thank You Points, but is never mentioned in Premier or Prestige application 24-month language?

I'd say your TYP choices are:

• Convert to AT&T Access and apply for Premier or Prestige any time you want
• Apply for Premier or Prestige first, and then right after that convert to Rewards+
• Convert to Rewards+ now, and wait at least 25 months before apply for Premier or Prestige (CIti is sloppy with counting months, so you should give plenty of "buffer")