Conservation stabilises an object and prevents further loss. Restoration returns it towards an earlier state. Repair makes it work again. These are different aims, and the same watch can require different answers in different components.
Conservation's default preferences are reversibility, minimum intervention, and documentation. A replacement part kept alongside the original, and a written record of what was done, preserves the object's evidential value even when function has been restored.
Some interventions are effectively irreversible: refinishing a dial, polishing a case, replacing an original balance staff that could have been repivoted, or re-engraving worn signatures. These destroy information that no later research can recover.
Others are routine and defensible. Cleaning old, gummed lubricant and relubricating correctly prevents wear; replacing a broken mainspring is normal practice and the original can be retained; a fitted crystal of the correct type is a wear part.
Judgement depends on the object's significance. A common utility watch used daily can reasonably be repaired for use. A rare signed movement with an original unrestored dial is a document, and running it need not be the priority.
Anything involving the escapement, balance, jewelling, or a striking or chronograph mechanism should go to a professional watchmaker. Practice on inexpensive scrap movements, not on the object you care about.