Does anyone know if IQ scores can change drastically?
Yes, they can change significantly over time but if your dd is performing as expected for the prior IQ, I would expect that it is likely they'll be similar. Worrying isn't going to change the outcome either, unfortunately.
My one child who was tested twice had scores on the same test 20 pts apart in only a year (lower the second time), but she is also twice exceptional and I suspect that seriously impacts her performance on things like that. Major reasons for changes in IQ would relate to 2e issues like my dd (either up after the LD is remediated or down due to anxiety, erratic testing in a 2e kid, etc.), regression to the mean, inflated scores early on due to enriched environment, or depressed scores early on due to poor environment.
It doesn't sound like most of those reasons would apply to your dd, so I'd be generally hopeful that the scores would be pretty similar.
Some articles on changing IQ scores and possible reasons:
http://aje.oxfordjournals.org/content/154/8/711.fullhttp://faculty.education.uiowa.edu/dlohman/pdf/Gifted_Today.pdfhttp://www.nature.com/nature/journal/v479/n7371/full/nature10514.htmlhttp://online.wsj.com/article/SB10001424052970203752604576641133332697322.html (this one reports on the same study as the Nature article above, but may give more complete info as access to Nature in full may be limited to subscribers.)
FWIW, someone is still gifted at any given point in time and not everyone's IQ changes just b/c substantial changes can be seen in some individuals. A child who performs as if she is gifted and has a prior IQ score that supports that would be high on my list of people to assume will test as gifted a second time (i.e. -- her odds of having a gifted level IQ score is likely much, much better than most.)
