Re-Indexing consists of changing the quantity of the tokens that make up the Investment Unit of a Fund. It may involve removing a token completely, incorporating a new one, or modifying the amount of another one, thus modifying the proportions or weights each token has in the Investment Unit and the Fund Value. See Investment Unit.
If the Fund were empty, with no deposits, i.e., the value of the Fund was zero, and there were no minted uLP, changing the Investment Unit would only be an update of the parameters of the Fund Datum.
But if the Fund were already managing the deposits of many Investors, i.e., there would already be many Investment Units deposited and uLPs minted and delivered to the Investors. The re-indexing process should also affect the actual holdings of the underlying assets of the Fund in such a way that the change would not impact the value of the Investment Unit, nor the value of the uLP, nor the value of the Fund. We cannot afford to defund the Fund during this process.
Changing the proportion of some Tokens will involve changing the ratio of others such that the ADA value of what is withdrawn is equal to the ADA value of what is paid in so that the total managed value of the Fund in ADA remains unchanged.
If you wanted to withdraw tokens whose value was 1,000,000 ADA, you would have to enter that same transaction ADA or Tokens with a value of 1,000,000 ADA.
A Re-Indexing transaction of significant value may be partitioned into smaller Re-Indexing transactions of lesser value. In such a way that each one affects all Investment Units but affects them in a small proportion. Instead of withdrawing tokens with a value of 1,000,000 ADA at one time, I can withdraw and enter ADA or tokens with a value of 1,000 ADA at a time repeatedly until the total of 1,000,000 ADA is covered.