The problem is, if I send to you 1 BTC, but it came from a 100 BTC coin, was there 1 BTC of "economic" transfer, or 99 BTC (which is the amount of the change transaction)? You and I know it was 1 BTC, but nobody else really can know that.
Isn't this similar with dollar bills or any fiduciary form of money ?
In a one-dollar transaction paid with a 100-dollar bill, we see 199 dollar units changing hands. It could be a 99 dollar tx (in smaller denominations) and a 100 dollar tx : no one could tell in the macro economics statistics of velocity, only the parties privy to the transaction.
With dollars there are two transactions, $100 to the merchant, than $99 to the customer. With Bitcoin there is 1 BTC sent to the merchant and 99 BTC the customer sends to their own address.
Anyway, the point is that change from BTC transactions would not be useful in determining velocity.