Hi, welcome to the board.
What exactly do you mean by anonymising the ID ?
A 7 digit number sounds pretty anonymous to me.
You could perhaps try modifying all the IDs in a consistent fashion - for example by adding some constant number to each of them, or by adding a fixed number of random digits to the end.
Both of these would CHANGE the IDs, allow them to still be sorted, and allow the original ID to be derived, if that was something that you wanted to do.