1, water freezes to rock hard ice. 

2. There is no legal procedure to terminate an adoption. 

3. The rights are the same but the treatment may vary.  As in all biological families as well. 

4. Those that adopted you  are your parents. It takes nothing to create a child and everything to raise one. 

 deidra:

what is your opinion on these questions:

1- blood is thicker than water

2- it's ok to change your mind and abandon an adopted child anytime

3- adoption means you enter the family but you will never have the same rights as a blood relative

and for the last question:
4- is it ok for you that an adopted child agrees to be registered again in his/her "blood' family register after his/her adopted parents died? yes/no, why?

Here's my take:

1. It all boils down to the actual relationship. 
2. In principle, it's not. It would be traumatic for the child. But technically, there are steps one may undergo within a certain period of time  (since the completion of the adoption process) if the child is to be returned to the institution that processed the adoption,
3. Same thoughts as @Lee C. The adoption process itself gives the adopted child the same legal rights as the biological children.
4. My answer would depend on the situation of the adopted child and the biological parent/s.

Follow-up:
If the situation is the same as Soo Jung / Jung Eun and Kyung Cheol's, I would definitely not force anyone to make decisions against their will. If Soo Jung chooses not to be registered,  then if I were Kyung Cheol, I will not force her. Instead, I'll settle on the consolation of being accepted into my daughter's life, in whatever way (that is much better than be considered someone who does not exist). If Jung Eun agrees to be registered, then good for Kyung Cheol. But this is without considering Hyun Jae and Mi Rae's predicament. The young couple's situation makes it all the more difficult to decide...

If you adopt a person, there are no backsies. There are no circumstances where you can legally (or illegally) abandon a child you adopt. You're their parent(s) legally, ethically, and financially. An adopted child has the same legal rights as a biological child. In the US, the adoption process usually changes the name of the child being adopted to their adoptive family's last name (but not always.)

"Blood is thicker than water" is an artificial and societal construct. In reality, once people in a familial relationship are all legally adults, any continuing relationship between them is voluntary. Your family, when you're an adult, may be composed of your nuclear family members, your spouse, and your friends, in any combination.

It depends on each person, different circumstances and different laws of different places. There can be no one or exact answer to this.

 deidra:

1- blood is thicker than water

This is the only sentence of orthodox people to make their point.

2- it's ok to change your mind and abandon an adopted child anytime 

I can say for sure about this thing that the law of any country does not allow this. its human emotion which are manipulated by people , situation or anything Because of which he leaves his biological  or adopted child . This is not only about the children, but also about the elderly, sometimes the elderly are abandoned.

 deidra:

3- adoption means you enter the family but you will never have the same rights as a blood relative

Under the law, both biological and adopted child have same rights, no doubt about it. as i said before different cases have different results . i know many people who adopted child and treated that child very well.  if i talk about my closed one has adopted a girl child even they have already a biological child, and the whole family treat that girl as princess. so Every person's situation is different, mentality is different.

 deidra:

4- is it ok for you that an adopted child agrees to be registered again in his/her "blood' family register after his/her adopted parents died? yes/no, why?

I'm not sure about registering procedure  their is any law or restriction to do that. what i know about that in my country after you reach adult age adopted child has right to search/find for their biological parents , even for this adopted parents can't stop them . and it allow them to choose with whom they want to live .

note : I'm not aware of Korean laws or society how work in this things .