i wonder what changed in the frontlines that suddenly made russia want/willing to use these bombs
One factor is just that they only managed to perfect their JDAMski kits only recently. The first strikes of the bombs at Avdiivka and Sumy seemed to me they have an experimental but test it in battle quality to it. The second factor is the deterioration of Ukrainian air defenses throughout the front lines as these are either attrited away by missile or drone, especially Lancet strikes, or forced to be relocated inwards to defend against Kaliber and Geran strikes.
The third factor is General Surovikin. While he did and is still leading ground forces in battle, Surovikin is also the head of the Russian Air Force. While in Syria, Surovikin led the VKS to do high altitude bombing against ISIS and Syrian rebels, which proved to be decisive in crushing them. These includes attacks against command posts, troop concentrations and deployments, infrastructure, supply, fuel and ammo depots. These attacks which no doubt brought their share of collateral casualties and damage to society and population, earned him the nickname 'General Armageddon'. He has brought the same MO to Ukraine.
Long wingspan Lancet strikes against radars and AA systems done their part. These particular variant did not appear until late summer or fall of last year and allowed the Russians to strike deeper behind the front lines. Russians have yet to formally disclose this Lancet variant to this day.