Can I move a local Asset to S3 and keep relations? I have a Assets folder (local) with 100 assets in it.

Now I have a new Assets folder connected to Amazon s3.

Can I just drag-n-drop my assets from my local folder over to my s3 folder and keep all relationships for those assets to my elements (entries)?
Yes you can. Just remember to add your new Assets folder as an assets source to your old Asset fields (fields that are "Asset" types) so users still can add/edit/change their assets.