Your Mom could probably save some cash by downgrading to the 100 Mbps plan. 100 million bits per second is a lot of data. If your Mom has an older router or device NIC, it may only have a 100 Mbps (Fast Ethernet) port, so she wouldn't get 150 Mbps.
If Cox removes a plan, Cox will automatically switch your plan to whichever plan would lower your bill the least. Meaning, if switching your Mom's plan to 150 would keep her bill unchanged, Cox will switch her to 150; however, the 100 plan may be $10 cheaper.
For example, I was originally on the Cox "Preferred 150" plan. When Cox restructured their plans a few years ago, Cox invented a 200 plan and "grandfathered" me onto this plan without increasing my bill. Nice, huh? However, during their restructuring, Cox had actually reduced the price of the 150 plan by $10. I just thought it was odd the 200 plan was NOT listed on their Speeds and Data Plans...but the 150 plan was still listed. Why'd Cox grandfather me to another plan? Money!