CHICAGO (CBS) — A rule switcheroo thousands of people by surprise as they tried to sign up for COVID-19 vaccines at the United Center.

CBS 2’s Marie Saavedra on Monday tracked down how the decision was made.

READ MORE: Illinois To Offer COVID-19 Vaccinations To Everyone 16 And Older

The city has emphasized that appointments will still be honored for anyone 65 or older who booked an appointment before Sunday – no matter where they live.

But for those in the expanded category 1B for vaccination who thought they were good to book at the United Center on Sunday, they found things quickly changed.

At 4 p.m. Sunday, thousands of people in extended 1B were likely at their computers in the ZocDoc website, selecting Illinois and answering questions to book vaccine at the United Center. Terry Stanton was one of them.

“I’m under 65 with a medical condition – I’m in that group – that we would have a shot of going to the website and making an appointment,” Stanton said.

The Oak Park resident thought he would be in. But he and few others knew about a las-minute change. He and anyone living outside the City of Chicago could no longer book through Zocdoc.

“I’m fine with waiting my place in line,” Stanton said. “My frustration was just how the decision was made.”

Since the announcement of the UC’s use, it has been promoted as a federally-run site open to all eligible Illinoisans.

So what changed?

Numbers showed since scheduling starter Thursday, less than 40 percent of appointments were booked by Chicagoans. Gov. JB Pritzker said Monday it was the Federal Emergency Management Agency’s call of at the end of last week.

READ MORE: Chicago Weather: Dry Sunday Morning With Isolated Showers Late In The Day

“Well the Federal Emergency Management Agency on Friday decided that they wanted to change the process,” Pritzker said. “They wanted to limit it to people who live in city of Chicago.”

We’ve learned Sunday morning that FEMA, the State of Illinois, Cook County, and City of Chicago had their final discussions – and then made it happen.

“I feel like with the changes made, agreed to by all, that we’re going to be able to fulfill our mission and focus which is on equitable distribution of this life saving vaccine,” Mayor Lightfoot said.

But this was such short notice, most outside the city only learned of this change when they tried to book. So in an already-panicked process of finding vaccine, Sunday’s switch only muddied the waters more.

“There’s a lot of discrepancy on how you get an appointment and sometimes it just seems totally random,” Stanton said.

Again, any new signups for the United Center site will be for Chicago residents only – for now.

The Chicago Department of Public Health said those who live in Cook County, but not Chicago, should go to vaccine.cookcountyil.gov. Those who live outside of Chicago and Cook County should go to coronavirus.illinois.gov.

The city also said questions about website functionality issues should go to Zocdoc.

Meanwhile on a call earlier Monday, CDPH Commissioner Dr. Allison Arwady referenced that there has been work behind the scenes to make sure some of the vaccine allotment from the federal government goes to suburban Cook County residents and outside of cook county.

MORE NEWS: Chicago Weather: Rain Tapers Saturday Night, But More Scattered Showers Sunday

We’re hearing details on what that means will be released this week, possibly as early as Tuesday.

Marie Saavedra