Must one mandatorily wear new yagyopavitam after shaving? As it is difficult to undergo whole-body shaving (Sarvanga Kshauram) in the cities, will it be appropriate if shaving is restricted to the face? For certain important rituals, able to shave only up to the his; is this appropriate?

Adorning new yagyopavitam after shaving is an etiquette that is still being followed. Since yagyopavitam comes into contact with foreign bodies, it is a practice to replace it after every such occasion. Though it is difficult to undergo Sarvanga Kshauram in the cities, several elders seek out those who can do Sarvanga Kshauram. Even in these times in certain villages, towns, or cities, people pay inordinate amounts asked for to undergo Sarvanga Kshauram. There is no harm in spending on this to observe a righteous ritual.
Shaving up to the hips is not a practice prescribed by the scriptures.