klaymen: You find new soldiers from time to time (scripts). In some missions you find a few people, a plenty of people or nobody at all.
The thing is, everyone can do everything. For example your US campaign hero, John Macmillan starts with combat skill at level 3, the other skills (engineering, mechanical and science) are at lower levels. That means, that he is a good soldier from the beginning (=he will do more damage in combat than someone with combat skill 0), but if you want him to become an engineer and build a workshop, just switch him and he'll do it (but obviously building built by him will be weaker than built by more skilled engineer).
Only thing you need to switch someone's job is the appropriate building (mechanics in workshop, engineers in storehouse, etc.)
So to answer your question: it doesn't matter if you lose all your builders, because your other soldiers can do it as well (after switching them to engineers). But here you have limited amount of people, so when you lose too many soldiers, it is better to restart the mission.
No offense, but maybe that could and should be explained
differently.
Any unit "type" can take on the role of another "type" ONLY IF you have a building that
will convert him. For example, a soldier can be converted into a mechanic by having him
enter a factory, and clicking the appropriate button.
HOWEVER.......only engineers can construct buildings. If you enter a scenario
without an engineer, you may not be able to convert anyone.
lowyhong: So if you don't restart the mission after losing 95% of your soldiers, you're screwed?
The game follows the plot mechanics. The Eon time machine casts persons
and materials imprecisely across time and space, so persons show up scattered.
You can rely on new people showing up all the time, unexpectedly.
However, it hurts, bad, to lose people in combat. You never quite have enough
personnel, and after developing and training someone across three or
four missions, it can be painful to see them get killed. You never know what
skill level the new people will have, or when they might show up.
I think the key is to save your game, very often. Insanely often.