Scaling

If you Scale uniformly (same amount on all three Axes), there shouldn't be a problem with Texturing here (as UV's are being used as Texture Coordinates), but I'd recommend Applying the Scale anyway.

I wouldn't use a Lattice to simply Scale an Object (although it's possible), but if you use it to distort a Mesh, weird things may happen to your Texture, the noticeability depends on (the regularity of) your Texture:

Different Texture Coordinates give different distortions, but often you can get away with it (especially with hand-painted Textures) if the Lattice deformations aren't too extreme.

If you Apply the Scale, you should be good to go. If you don't Apply it, unexpected behavior should be expected.

Yes, you can change things as long as you apply the scale and don't leave things all wacky in their transforms. Try to keep things clean, that way you avoid headaches down the line.