Some incorrect info in the first response...
Ivy league does not offer athletic aid. But, for school that do, they can not immediately cancel aid for entering the portal. A school can cancel aid if an athlete becomes ineligible, or voluntarily quits (not the same as entering the portal).
While a school can not immediately cancel athletic scholarships, they can cut an athlete who enters the portal from the team (up to the coach/admin). Very likely that the coach will also tell the athlete they will not be renewing their scholarship at year's end,even if they don't transfer.
For undergrads, I think it likely that the coach will remove an athlete from the team for entering the portal, but every situation is unique.
For graduates (especially in the Ivy League) schools will likely have a different approach to the portal, but again, every situation is unique.
Long story short--athletes can enter the portal at any time, and even transfer within the same conference and be immediately eligible. But, if you enter the portal and the offers or opportunity is not what you wanted there is still nothing to stop the coach at your current institution from removing you from the team and cancelling your scholarship at the end scholarship contract's term.
Best approach; entering the portal is not a decision to be taken lightly. Enter it only once you are positive you want to transfer. There are consequences to decisions.