Anyone that has entered the portal can transfer any time until they are attending classes at another school.
It is possible to still transfer if you didn't enter the portal, but the NCAA instituted new rules, for those kids.
"The Division I Cabinet adopted a proposal that penalizes programs that sign a transfer student-athlete, add a transfer student-athlete to a roster, or allow a transfer student-athlete to participate in athletically related activities before the student-athlete is entered into the NCAA Transfer Portal.
Penalties — which are automatically triggered — include a suspension of the respective sport's head coach for 50% of a season and a fine of 20% of that sport's budget."
There isn't a school out there that would accept those penalties.
I don't think it would stand up in court, but it hasn't be challenged yet.