I really wouldn't worry about it too much, and I'd let them retest. Coming at it from a different angle, three years is a time-frame that is used for re-testing IQ and achievement for other reasons - for kids who have IEPs and for kids who need accommodations for LD etc on high-stakes testing such as College Board etc. Chances are your dd's IQ hasn't changed outside of whatever the measurement error of the test is (which should be just a few points), unless the original estimate was inflated to begin with.
I'd look at it from this perspective - you've successfully advocated for a G&T program at your dd's school - that's wonderful! (Congratulations, btw!). There has to be some way of screening and qualifying children to get them into the program. What would you think if you were one of the parents of other children who haven't been skipped and who are being screened - would you wonder why one parent's child was automatically allowed in without the screening? What if you were the principal and suddenly this parent who'd pushed pushed pushed for the program thought they should be able to have their child skip the screening - would you wonder if they didn't doubt just a bit that their child might not qualify?
The two things I'd wonder about are - what is the IQ cut-off for the new program, and if a matrix of data is used such as including achievement testing, a different type of ability test, teacher recommendation etc, are kids who come in just a bit below the IQ cut-off going to be able to get in based on an overall qualifying matrix?
If your dd did happen to come in below her previous scores, I think you could take all the reasons you might be worried about testing now (having an off-day when testing etc) and turn them into reasons to advocate that the second test isn't reflective of her potential (along with past performance in school etc). OTOH, if her previous score was already below the cut-off or she's very near it, that might be more difficult to counter.
And otoh.. what exactly is the gifted program going to be? A pull-out or a full class grouping? Either way, if she doesn't get in, she's not going to be retained in the same grade next year is she?
Best wishes,
polarbear
