Selling damaged units and replacing them

Well, this is certainly interesting. To me, I only sell the unit if it requires 300 to 400 crystal worth of repair time. Any cheaper than that, I would ignore. I also worry about selling vertigoes if they are at a high level and I have few crystal... do it wrong and the vertigo returns with a lower level. Sometimes when I have 2 vertigoes of either the same or similar levels, I'd sell one just to upgrade the other.
I'm just simply looking for opinion as selling units, replacing them and upgrading them feels really risky as you can sometimes have insufficient crystals after an attack.