Four friends I met in Cambodia all left the US after they retired and had no problem for the decade plus each of them were abroad receiving benefits.

In most cases, all you have to do is fill out the SS-7162(proof-of-life) form each year and snail mail it within two months of receiving the form back to the SSA.

There are any number of reliable international courier services to choose from.

Apparently you can also complete proof-of-life with the SSA by phone, although the guys I knew filled out the form and mailed it in each year.