@allwync88 sadly none of us are doctors, we are patients like your mom. That being said I can try and translate a little. Lytic lesions are where the cancer causes a hole in the bone. My husband has this type of lesions in his bones so I am familiar. Your oncologist probably just read the read the report because the radiologist is the expert at reading the scans and they write a report for your doctor.
I have received quite a few of these reports over the years and it sounds like they found a change in your moms cancer pretty early, and are changing the drugs since the old ones are not being as effective anymore.
I cannot really give you any more information about it, since all I can really do it translate what you told me a bit. I know how scary this is, I hope it helps.
How are you feeling about all of this, how is your mom feeling? Has she started the new regimen yet?