Is it true that human's DNA can change in several years due to spinal fluid transfer into someone spinal chord
No, this is not true. Your DNA generally stays the same throughout your life, although there is some slight rearrangement in various tissues to perform special tasks. There are some mutations in cells exposed to radiation and toxins, but this effect is minimal when a sample is averaged over many cells.
A sample collected by the police and stored properly is limited mostly by the amount of sample collected and the environment that sample was exposed to prior to collection. (For instance exposure to high levels of heat or chemicals such as bleach.)
Is it true that human's DNA can change in several years due to spinal fluid transfer into someone spinal chord
A person's dna does not change. If the samples were not collected or stored properly then they can become denatured and, therefore, useless but this change it not due to mutation.
