In general though, it's pretty common these days for games to not be save game compatible across different versions. It's unfortunate, but such is life.
I'm not sure about that, it's been my experience that very few games make their save games incompatible.
It completely depends on the underlying structure of a game and what all data gets inherited or absolutely specified. I don't know how Sins works, but if you have a save game where it absolutely specifies that your frigate has 2 armor, 20 of 600 shields, and 400 of 400 hull points then you instantly have a problem if the armor, shield, or hull points change. Now if that same save game said you had +1 armor, 0.03 shields, and 1.0 hull points, then any change to the armor, shields, and hull of your frigate will automatically adjust for gameplay changes. 2 is an absolute value that has no idea what the default should be, whereas +1, 0.03, 1.0, or some other multiplier is a relative value. It requires the original armor number for that vessel to be calculated and then instanced referencing those original numbers and then applying the modifiers to create this specific unit.
It's all a matter of underlying structure. I've played games that invalidate their saves and other games that don't. I've had some funny things happen with save games that aren't invalidated and a lot of normal experiences too. When you're targeting lower-end computers it's highly likely that absolutely saving everything will tax system resources less and your customers will perceive good performance when saving and loading. If everything is instanced and referenced then there's exponentially more stuff to calculate and perceived degraded performance despite a much more flexible system. It's a design decision pure and simple.
