I first saw mention of this change of language a couple of weeks ago.
following is dated 5 Jan 2022 in search results:
Updated 16 Jan: CDC: Stay Up to Date with Your Vaccines
Get Vaccinated and Stay Up to Date
Up to date means a person has received all recommended COVID-19 vaccines, including any booster dose(s) when eligible.
Fully vaccinated means a person has received their primary series of COVID-19 vaccines...
CDC recommends that everyone 5 years and older get their primary series of COVID-19 vaccines, and receive a booster dose when eligible...
Pfizer-BioNTech
Booster Dose
Everyone ages 12+ should get a booster dose at least 5 months after the last dose in their primary series.
Teens 12–17 should only get a Pfizer-BioNTech COVID-19 Vaccine booster
Everyone 18+ should get a booster dose of either Pfizer-BioNTech or Moderna (mRNA COVID-19 vaccines)...
When Boosted
A person is considered “boosted” and up to date right after getting their booster dose...
4) The primary series of this vaccination includes a third dose for people ages 5–17 years with moderate to severe immunocompromise. The third dose occurs 28 days after the second dose in the primary series...
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/stay-up-to-date.html
AP’s Zeke Miller feeds into the “up to date” line:
5 Jan: White House: Press Briefing by White House : COVID-19 Response Team and Public Health Officials
MODERATOR: Let’s go to Zeke Miller at the AP.
Q Thanks for doing this today. For Dr. Walensky, I know (inaudible) questions about this over the last several months, but given the data you’ve discussed today about the benefits of booster doses and Dr. Fauci’s comments yesterday about, sort of, potential changes to CDC policy: Is the CDC going to change its definition of “booster doses”? Or —
And then, for Jeff: For people who are not fully ***up to date with their vaccination — meaning they’re eligible but haven’t gotten a booster — should they be expecting to see, you know, that some of the prohibitions that people who are not fully vaccinated — you know, whether it comes to travel or employment with the federal government — that they would be subject to some of those prohibitions down the line?
MR. (JEFF) ZIENTS (White House COVID-19 Response Coordinator): Dr. Walensky first.
DR. WALENSKY: Yeah, super. Thank you, Zeke. So, individuals are considered fully vaccinated against COVID-19 if they’ve received their primary series. That definition is not changing.
But consistent with how public health has historically viewed or even talked about how we recommend vaccines, we are now recommending that individuals stay up to date with additional doses that they are eligible for. And we have now available how you can stay up to date with your COVID-19 vaccines based on what vaccine you have received and what age group you are in. And that is available now on the CDC website.
Back to you, Jeff.
MR. ZIENTS: So, Zeke, to repeat what Dr. Walensky just said: Someone is considered fully vaccinated if they receive their primary series of vaccines. So, if you think about the different requirements — I think you mentioned travel, the OSHA and CMS rules as other examples — that has not changed and we do not have any plans to change that.
I do think it’s really important to recognize the vast majority of hospitalizations and deaths are among the unvaccinated Americans. And completing the primary vaccination series is clearly a critical step to prevent severe outcomes, with boosters, as Dr. Walensky said, giving the highest level of protection. So, we will encourage everybody to get vaccinated and, when eligible, boosted.
As to the definition, someone is considered fully vaccinated if they received their primary series of vaccine.
https://www.whitehouse.gov/briefing-room/press-briefings/2022/01/05/press-briefing-by-white-house-covid-19-response-team-and-public-health-officials-77/