Is there a sin upon whoever names their child "Al-Jayer" (with a ya not a hamza), and if so, is it obligatory to change the name?

"Al-Jair" is among the disliked names because it is an active participle of "jara" (جار), meaning "he deviated from the truth." The fact that it is spelled with a "ya" (ياء) and not a "hamza" (همزة) does not change this, as a hamza can be replaced by a ya. The correct course of action is to change this name, just as the Prophet, peace and blessings be upon him, changed the names of some Companions due to their undesirability.