Put simply: You only gain the abilities and benefit of one class at a time. So, yes, if someone is a Level 4 Soldier and they switch to being a Level 1 Expert, they can no longer use the Soldier abilities such as Weapon Mastery, Parry or Disarm until they switch back to being a Soldier.
Thankfully, a Starfallen can change their class at any time so long as they're in the comfort of their own home (or, at the very least, nice and safe at their camp for the night), so there's no need to worry about needing to start over just because they've stepped off of their highest-leveled class for a moment.
It bears repeating, however, that Mastering a class gives a permanent ability unlock that will be usable regardless of which class the character is currently using. For example, mastering Gladiator would allow use of the Counterattack ability even if the character switched to Expert.
no subject
Thankfully, a Starfallen can change their class at any time so long as they're in the comfort of their own home (or, at the very least, nice and safe at their camp for the night), so there's no need to worry about needing to start over just because they've stepped off of their highest-leveled class for a moment.
It bears repeating, however, that Mastering a class gives a permanent ability unlock that will be usable regardless of which class the character is currently using. For example, mastering Gladiator would allow use of the Counterattack ability even if the character switched to Expert.