OSIsoft SDK and API are the two different things. If changing the API to SDK, your class structure might not need to change, but the functions in it have to be changed into SDK way. When you installed PI SDK, there is the help file in the ../pipc/help folder, and also PI API's help file in there. You could compare them, and it should be not difficult to find how to change your API function into SDK.
OSIsoft will suggest customers to use SDK instead of API in the future. Currently, the new SDK, named AF ADK, is coming. If interested in this, you could go to v-Campus download center to download one to test, and I believe you will like this as it is more easy to use with the good performance.
As Xi mentioned, PI SDK and PI API are completely different and PI SDK is in no way a wrapper around PI API. In terms of value of updating it to use PI SDK, I would consider if the new features in PI SDK will be useful for you, for example
My personal view is if you have a need to add on new features in your library, I would recommend starting with PI SDK instead of continuing with PI API.AF SDK is another option that has equivalent feature as PI SDK, in terms of reading data from the PI Server. But it is writing in .NET, which means the output library that you get is also a .NET dll. This might be a problem if you are primarily writing this library for use in VBA environment.
- PI SDK is HA aware whereas PI API isn't.
- PI SDK can utilize Windows Integrated Security to authenticate with PI Server, but PI API is limited to using PI Trust.
- PI SDK is object oriented and probably easy for further development compared to sticking to PI API
Just to add on to my own statement, it is possible to write a .NET library with a COM wrapper such that it is possible to be used in VBA, but obviously there are some additional handling required. (hint, see the AF Wrapper for PI ProcessBook example in the Download Center)