Restoration and rebuilding can be confusing terms with different meanings to different people. I prefer to consider “Rebuilding” to mean replacement of all minor parts, including key tops, hammers, hammer shanks, whippens, strings, tuning pins, agraffes, dampers and all key felts. Rebuilding might include repairs or restoration of the original soundboard or complete replacement of the soundboard and bridge with a custom build board. A properly rebuilt instrument is usually difficult to tell from new.
Restoration should include repairs to all original action parts or only replacing very worn parts, such as the hammers, while saving what original parts that are still functioning well. Key bed and key felts and critical action felts could be replaced as these contribute to smooth and soft playing.
Treble and bass strings can be cleaned and polished, the soundboard can be cleaned and waxed or treated and the plate can be polished to greatly improve the appearance of the original finish without a major expense.
A few simple adjustments to the action along with cleaning and lubrication can make an original instrument play better than it has since it was new. Restoration can greatly improve the look, feel and sound of an instrument at 1/10 the cost of a new or rebuilt piano.