Michael and Sarah,
I read this thread and your other one. It's probably best to keep all related posts in one thread, such that it keeps all information in one spot. I cannot imagine there would be any issues with applying for the Ancestry visa before the student one expires, but you'll probably want to get on that this week if she's going to Canada at the start of October.
If she doesn't already have long-form birth certificates for herself, her
UK-born grandparent and the relevant parent then she'll need to order these ASAP. They generally take 2-3 weeks and you'll need them when you enroll for biometrics. You can Google: "_____ birth certificate" (subbing out ____ for country/province) to get the forms necessary to get these certificates. It also helps to have other supporting documentation as recommended. The more you can submit the better.
When you've got all the documentation, go to the Visa4UK website (this forum blocked the link and subsequently deleted the nice post I had written up), but there's a link to it in the UKVisas link provided by agnelson.
Then click on the blue "Apply now for a
UK Visa" button. It might say "Down for Maintenance," since it just said that when I tried it now, but at one point, it'll let you through and you can fill out the questions from there. Just make sure you specify "Ancestry Visa" when it asks you what type of visa you're applying for. I can't remember if you have to go into "Other" or not. From there it should be a straightforward process.
I don't know if she'll have to make an appointment for biometrics since she might have done that with the student visa. If she needs to do that, then things might get complicated. She'll have to go to Canada for the appointment, give them her passport, and wait around in Canada until she gets the passport back. I was quite lucky in that I got mine back in 4 business days, but the consensus seems to be about 2 weeks. It's generally a bad idea to book your
flight until you get your passport back. This might make it difficult to ensure she's back in the
UK at the end of October, but I don't know exact dates or how busy their offices are. I'm also just speaking on the experience of getting my Ancestry Visa about 2 months ago.
Good luck.
Jim.
P.S. If Sarah does have to go back to Canada and has her choice of biometrics offices, I'd suggest Ottawa since they send your passport there anyways. It won't make more than a day or two's difference since everything is shipped priority courier, though.