My wife and I became “federal voters” in the spring, and were able to vote in both our state’s primary election, and to cast primary ballots for candidates for federal office (which may vary depending on your state) — as we had to declare our intention eventually to return to our state, in order to participate in state elections. (Federal voters also get to vote in the General Election.)
We accomplished these changes by contacting our former county board of elections, submitting signed encrypted documents electronically (rules may vary). We were then able to cast our primary ballots by shipping them to a friend in Florida by FedX, who took them to a local post office where they were hand-cancelled (to verify the date of their entry into the mail stream). Don’t rely on foreign mail to deliver any of these documents in either direction, as they likely will not arrive on time.
If you want to attempt this, I’ve linked some details, gleaned thru or from League of Women Voters (by referral, to US Vote Foundation) and the federal government to point you in the right direction, but you might best contact your former board of elections to get an application by email. (We wanted to avoid partisan recruiting by party operatives, as my sense is that they sometimes engage in pesky fund-raising schemes.) Be alert! Don’t give your personal details away online, without encryption. Know the destination, with certainty. More on federal voting from Federal Voting Assistance Program.
NOTE: NEVER send ballots to a board of election by private carrier, such as UPS, or FedX. They will be invalidated. Ballots must be carried by US MAIL, and need a valid, readable postmark — which can only be assured if someone submits them for hand cancellation. Over 550,000 ballots were invalidated for various reasons in the last federal election cycle. Don’t let your electoral voice be snuffed by casual handling!