Do NOT call. This is basically a guaranteed way to get your family moved back to standard economy.

In general, if you cancel then your family will remain in their existing seats. There are possible event that can occur (some of them obvious, some behind the scenes) that can cause them to be bumped back to economy - but in general these are unlikely, and get even more unlikely the closer you get to the day of the flight.

If you go this route there's no guarantee that they won't end up at the back of the bus - but realistically the odds of them keeping their seats are very good to excellent.