Changing the tree variety, or species, can be done by budding or grafting.  Here we are putting an apricot onto a Lovell peach rootstock.  My experience with field budding has been sketchy.  This year's experience with grafting has been very good, with over 90% success.    In the future, I will choose grafting over budding whenever possible.

Apricot grafted onto peach

We covered the newly emerged buds on the grafted wood with paper bags to protect them ahead of an herbicide application.

Paper bag to protect buds from herbicide