It is quite easy to change domicile by deciding to stay here for ever even if you lose your job, your partner dies, or your mom back home gets ill; plus and telling all of your friends and family that this is your choice (it does happen, say you have kids who were born here and are now at secondary school here and so you expect to stay to watch yoour family grow, whatever happens).
However all recent case law demonstrates that changes from a domicile of origin to a domicile of choice are in practice very very difficult if one retains connections in one's previous domicile.
There is however a common misunderstanding in Matt's post in that no-one is domiciled in 'the US'. You are domiciled in a State within the US.
To get the favourable UK tax position you need to claim both to be non-UK domiciled and to be taxable on the remittance basis.
This usually needs some advice because the Revenue's forms are designed to confuse.