According to Rolex, change anything and it's a rep, including the strap. This is being contested in court, I believe.
Realistically, if the movement matches the case, it's a gen. If the movement and case come from different watches, it's a frankenwatch. This is my understanding, at least.
To be a trademark infringement, any part with the company's brand or trademark on it that wasn't produced by the company is an infringement. This means aftermarket crowns are possible infringements, as are replacement Rolex crystals with the crown engraved on them.