Do set up all your accounts BEFORE you leave the US. Like many others, Fidelity won't open a new account if you have a foreign address, so they won't turn my 403b into an IRA, with the usual blah-blah about security, Patriot Act, etc.
Don't know if it matters but my 403b is (obviously) an employer plan.
Also, not totally on point, but if any of your accounts are 403b or 401k, and you have any taxed contributions in your account, study first the implications of rolling over to an IRA. They rollover as untaxed. And if relevant, any 403b grandfathering is lost.