I have no idea if CO will treat the payment differently because it comes from TY points as opposed to normal/direct payment channels. Assuming they treat it the same the process goes something like this:

• You pay the change fee as an additional payment; it is not deducted from the value of the ticket.
• The change fee is paid when you book the new flight, but you have to call to cancel the old one prior to the old departure.
• You must book a new ticket under the same name as the original ticket. This new ticket can be for any price.
• Should the new ticket value be lower than the original ticket value the residual credit will be issued as a TCV.
• The residual TCV value can be used to book as many tickets as you want for any passengers (not just the original name).

In your case you'd pay the extra $150 (or whatever the change fee is) out of pocket, and you'd have to redeem one for yourself first, but after that the TCV value can be used to buy tix for anyone.