If DD has accepted offer,

[bubble2000]

And makes the grades

Can she still pull out and go into clearing, for example, for a different course or university?

Yes

Yes, although it would make sense to secure the second offer verbally first , then self release.

Is there a deadline for this? Would she only be able to see which courses have space on tge day of clearing? Is it worth contacting possible universities now?? She wants to change but doesn't want gap year 🙈🤷‍♂️

Is it a complete change of heart or just a sideways shuffle?
If it’s just a sideways shuffle she could try phoning the unis directly and asking if they would consider her on her alternative course, if they say yes, amendments can then be made by them to ucas, I think?

I think its worth contacting universities now. If its the same universities she can apply for a course change. There's details on UCAS website.

https://www.ucas.com/undergraduate/after-you-apply/making-changes-your-ucas-undergraduate-application

DD did a course change at one uni for an insurance offer (just single to joint with a similar subject to get a lower offer otherwise was same as firm offer) and just applied for course change via UCAS and the second offer came through about a week later. I would phone or e-mail that universities admissions team. There's something called Extra too, https://www.ucas.com/faqs/what-extra-and-how-does-it-work

If it is the same uni she might be able to renegotiate the course otherwise she can withdraw and try Ucas Extra or wait for results day. How different a course is she wanting?