An interesting exercise is to determine the optimum time of selling the home: now or when they're ready to buy their replacement home.
If they intend on traveling for more than three years, then, it may be a good decision to sell prior to the sojourn to avoid paying taxes on the gain in their home. IRS allows for a temporary rental of a principal residence while still keeping the $250,000/$500,000 capital gains exclusion intact. A homeowner must own and use a home for three out of the previous five years which means that it could be rented for up to three years, but it would need to be sold and closed before that three-year window expires.
If the travel will be less than three years, there is an option of selling now or later. Using the example below, the homeowner sold the home, paid their expenses and invested the proceeds in a three-year certificate of deposit until the replacement home was purchased.