1) How can someone that has a JTWROS Brokerage account "break" that account, or change that account to another type, like for instance Joint Account in Common. The goal is to protect the heirs from the "survivor" in the JTWROS, since with this type of account the survivor becomes automatically the owner of the 100%.

2) Since any party in a JTWROS brokerage account can dispose of the assets, is it possible that one of the holders sell 100% of the investments and then transfer those proceeds funds to the checking account in that same bank and then finally write a check withdrawing 100% of these proceeds?

Thank you,

If the brokerage rules will allow, each person on the account will sign to have the account changed. It can not be changed without approval of both people.
In some cases a new account will have to be opened and the funds from the old account moved to the new account.
As long as both people are in agreement, it can be done. It can not be done, if one person will not agree.

It will depend on the rules of the account, if the account requires approval of both people for withdrawal? Or only one person, Each is correct depending on the rules of the account.
If one person does it, without knowledge of the other, while it will not be a criminal theft, it could be a tort issue of conversion and make the one who took the money libel in civil proceedings.

Actually, a JTWROS would pertain to real estate, not a brokerage account. The idea is similar, however. If one of the owners or "holders" withdraws the funds and transfers them to another account, yes, the funds would belong to the holder of the other account. Then, if this holder were to die, the new account would be a part of his or her estate.