You can't transfer call signs per say. You can pick up one that was previously issued if has not been renewed for more than 2 years under the vanity license system.
There is an exception for family members such as your grandfather's call sign if he's deceased (silent key). In that case if you can provide some documentation to the FCC you can use his call sign immediately, no need to wait the 2 years for it expire. Be aware that if his call sign was for a higher class than you hold you might not be able to claim it.