Talk to your insurance agent about the different types of coverage you have on your policy. Ask for a copy of your Insurance Declaration Page and see the different types of coverage you have and the policy limits for each type of coverage.
Underinsured Motorist coverage (UIM) is there to protect you in situations where the person that caused the accident is underinsured and there is not enough coverage to pay for your medical bills. You must obtain the policy limits from the person responsible for the accident before you can get your Underinsured Motorist (UIM) coverage from your own insurance company.
Uninsured Motorist coverage (UM) is to protect you from uninsured drivers and hit and run accidents where you can't identify the vehicle that hit you.
You cannot move coverage around. The policy limit for UIM coverage is all the insurance company is required to give you for settlement purposes. That is true for all types of coverage you have on your policy. Be sure you understand what types of coverage you have and what it means for you and your family.