MATLAB: How to load, update and save data neatly

I have a variable saved to a .mat file which I wish to load in, update (extend) and save out again to the same file.

Obviously I can just call load with no output argument and then save. But people generally seem to say that

s = load( someFilename );

is far better than using the no output argument form. This I understand.

What I am finding difficult to do neatly is the saving back of my updated data. I can update easily as:

s.myVar = someFunction( s.myVar );

but then I can't just do:

save( someFilename, 's' )

as that will leave me with a structure in my file which would not then match the original format. I also can't do:

save( someFilename, 's.myVar' )

as this is clearly nonsense.

So do I really have to do the following?:

myVar = s.myVar;
save( someFilename, 'myVar' )

If so this seems to obviate the purpose of calling load with an output argument in the first place.