Is there legal paperwork to change the POA designation and let us switch places and me be primary and my sister secondary?

Nelly, read those papers again, because a Will is for after someone dies, when there is no "medical need" to attend to.
Perhaps you are looking at an attachment to the Will that designates a POA for health care?

I have a will with my 98 year old Grandmother that lives in New York naming me as the executor of her estate and medical needs if my dad who is her only son dies. My dad died 2 months ago. If my Dad who is her only child had Power of Attorney do I need to get a power of Attorney to take care of her health and financial needs?

Your sister and you can't just change places on the POA. For you to become primary POA, she must resign.

I don't know how a primary POA legally and officially resigns. That would be a very good question to ask the lawyer who drew up the POA. If that lawyer is no longer available, then find a lawyer to ask.

You don't have to file for guardianship to become primary POA as the secondary POA if she wants to resign form being the primary POA.

Yes, there is paperwork involved to change POA. Get a DPOA from your mother (if she is competent) so you can sign papers in the nursing home.

Depending on the wording of your POA, if your sister is finding this burdensome she should be able to resign her POA and allow you to take over as the named secondary. She would thus abdicate all control of your mother's finances to you, of course: would you both be comfortable with that? And, don't forget, you need to think what happens should you no longer be able to manage the responsibilities.

If, on the other hand, it is you who is finding it a pain in the neck to have to contact your sister every time something wants doing, you can bolster your patience by reminding yourself that it isn't your job to get these things done promptly so they will just have to sit on the 'to do' list.

And if the NH is grumbling, they can grumble at your sister. Not your fault, not your problem.

Now that she is not able, any changes would have to go through a court. Uncontested Guardianship is relatively simple, but takes time and the legal fees for us were about $3K, paid from mom's assets.

Contact your local Area Agency on Aging or Bureau of Senior Services, they have staff that should be able to give you information regarding this issue.

You will need to look at the wording in the POA to determine if one could resign as POA in favor of the other, but still be able to be a secondary POA.

She is no longer able. She has dementia and is in a nursing home

If mom is still able to sign, physically and mentally, then have her sign a new POA changing the order in place of the old one. The new POA should revoke the old one, but if you have provided copies to anyone, they should be notified and provided with the new POA. A revocation could also be done and recorded.