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Well I'm still not sure what you're stuck on. How can you be updating something if you don't know what it is?
struct foo
{
type thingyouknow;
type thingyoudonotknow;
type...

Sure, if you know what the members are.
&somestruct.somememberI don't understand why you think you need to use memcpy if you know what every one of the members are. Just assign values directly, as...

Be lazy:
union
{
struct thingineedtoupdate foo;
struct whateverithinkiamdoing bar;
} baz;

baz.bar = doingstuff;
dowhateverithinkiamupdating( baz.foo );