Can the result of a bone marrow compatibility test from a sister 13 years ago change?

Hello Doctor,

I am writting this question on behalf of my mother.  She was asked to be tested once again and does not understand why she has to be retested given that the results 13 years ago indicated that she was not compatible.  However, since that time my mother has become older and sicker and is worried not just about the patient (her brother) but also the risks that are or may be related to herself.  Can you please help us by answering the questions as soon as possible, time is scarce. We were told that the results from a blood compatibility test would only be known in about 2 weeks, and it is unknown how much time the patient has.  Please understand that all the information that has been provided to you, Doctor, is all that is known to us.  Thanking you in advance for all your kindest help and attention.

If a sister has already tested negative for a bone marrow transplant, can she become positive 13 yrs later ?
Has the technology changed or has there been new medical breakthroughs that would permit a sibling who was initially tested negative for compatibility for bone marrow 13 years ago, be able to test positive today in 2008 and provide such bone marrow?

Also, if the sibling has already tested negative and he/she decides to redo the test than mathematically speaking shouldn't the test yeild the same result (i.e.: test negative)?

Moreover, if someone has sleep apnea, notably obstructive sleep apnea (OSA) in its severest form, and he/she wishes to donate his/her bone marrow (providing that the blood test is positive), how would this person be anaesthetised? Does this person need a sleep apnea machine, hence CPAP, attached to them during surgery? Is a surgeon able to perform surgery on this person? If so, how would he/she go about it given the sensitivity and gravity of this situation? What are the risks/dangers involved for such a person?

If someone has DVT, blood clotting and takes cumadin (coumadin) for life, can this person be tested ?

Thank you.