I think this is also what you are saying, but to clarify....let's say they have an item of one girl's clothing with a very tiny spot on it that contains a sample of DNA that is potentially the offender's DNA. The concern in years past would be that the sample would be "used up" in the first round of testing and there would not be enough of the sample left to do other types of analysis, should other technology become available in the future. This is not such a big concern today as there are amplification methods that will give you "enough" DNA to work with. Once that unknown profile is digitized and in the criminal database, you don't need to keep going back to the original sample to perform other rounds of STR matching (which is what CODIS and state databases use).
The only area of question would be, no matter the physical size of the sample being worked with, was it deteriorated/degraded, or was it complete? Were the STR loci that are used in CODIS represented in the sample that LE have, or had some of them dropped out through degradation? If all 20 aren't there then you are dealing with partial DNA and in the future you may want to look at how the original analysis was done, see if it can be done again at some point (perhaps by a private lab that uses different algorithms), look into partial matching IF it's legal, etc.
Nowadays there is, of course, genetic genealogy which doesn't use STR genetic segments, it uses SNPs. It's different than what CODIS uses and the labs that analyze these would need their own sample to work from, from my understanding. Nevertheless, they can also work with very small sample amounts due to aforementioned DNA amplification techniques.
All MOO.
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