using +20 Lesser Heart of Wood for LR'ing less than 20 levels

[vasfii]

Is it possible?

if yes, suppose you LR a level 15 character with a +20 LHW. would the 5 levels you havent used go wasted?

[Lanhelin]

Yes and yes. You can use the +20 Heart whenever you want, but you cannot take more levels you actually had before LR, so the remaining levels to 20 are lost.