There is no workaround to this issue at this time. It is a current limitation of the Excel Add-In.
1. You can not modify the name of a type if your hierarchy is already populated. This information is documented here: https://help.rallydev.com/portfolio-admin#type
In that page it says: "You can modify the name and ID prefix of a type, but not the order in the hierarchy".
When changing a portfolio item type's name then Agile Central will replace its REST Endpoint under /PortfolioItem with another. If a change is made to a Portfolio Item Type, edited "Investment" and changed it to "BusinessInvestment" then the /PortfolioItem/Investment endpoint will be replaced with /PortfolioItem/BusinessInvestment.
Existing objects of "Investment" will be lost. They will not show up in Agile Central as well as not be found by WSAPI. The objects are already under /PortfolioItem/Investment and once replaced they will no longer be accessible.
If you have no previously created objects of that type (Investment) then the conflict can be avoided by not creating a type of Investment.
2. The best way to avoid this issue is to NOT use names in your Portfolio Hierarchy that match any existing REST Endpoint, in this example: not to use the 'Investment' string/name. There are two issues around that:
a. If you already have objects created under that name (see #1 above).
b. Agile Central adds new objects along with their matching REST Endpoint names. Even if you choose hierarchy names that are safe, you must consider that these names will be 'taken' later. So, for example, if you created your hierarchy with the word "Investment" even before Agile Central had its own object then you would be okay until Agile Central introduced its own object. Once introduced, the conflict will happen.
Therefore, the best way for you to secure your portfolio item types is to prefix them by "PortfolioItem" or another prefix, i.e: Instead of "Investment" , use: "PortfolioItemInvestment" , instead of 'Epic' use: "PortfolioItemEpic" and so forth. The implication of that though is that these will now be your real names of your types, they will be displayed in all screens and reports.
3. If you are in a situation where you already have portfolio items, also linked with User Stories and other artifacts and such a conflict arises then you may want to manually create your additional portfolio items and not use the Excel Add-In. The only other alternative will be to export your data, then renaming your conflicting portfolio item type, then importing it under the new name, then proceeding with your original import. However, this may be risky, especially if there is a large amount of data.
Should there be doubt about any on this a support case should be opened.
To know all of CA Agile Central's REST endopints, you can refer to: https://rally1.rallydev.com/slm/doc/webservice/
Under the "Object Model" section you will see the list of all REST Endpoints of the given workspace you're examining.
This page is specific to a workspace which is highlighted at the top-left corner. To change the workspace, log into Agile Central, then change to the workspace you are exploring, then click that link.