What if Teeth Move After Mold Taken but Prior to Installation?

You state that you had molds taken and braces will be put on soon.  What method?  Are you doing Invisalign or brackets?  If it is the latter, then there is nothing to worry about, the brackets are placed directly on the teeth where desired.  If it is Invisalign, there is STILL nothing to worry about, as the teeth may move some but not enough to compromise the treatment.

You'll be fine.

Impressions for molds were probably taken for one of two reasons. The first is for the purpose of diagnosis and treatment planning. The second is for fabricating the braces. If the teeth shift slightly, the first type of mold is still usefull. However, a mold for making braces has to accurately reflect the position of the teeth at the time the braces are to be placed. Since teeth are constantly shifting, as a general rule it makes sense to allow no more than a week or two to pass between taking the impression for the "braces" mold and the placement of the braces. I suggest you call your orthodontist's office and inquire as to whether the mold should be retaken prior to your next scheduled appointment. 

The molds are taken at the beginning to diagnosis your problems and any movement is not likely to have an effect on the treatment plan.  With Invisalign, the molds are used to make the aligners and even if there is some minor movement, the aligners will still fit....only major tooth movement between the impressions and the start of treatment would effect the fit of the aligners.