This.
THere aren't that many CL rooms so if you really want one, book one.
If you are travelling at a "slow" time you might have luck upgrading at check in but it will most likely be at full rack rate.
At check in, cost of upgrade is generally difference between your current cost and the Rack Rate for the upgraded room. Not generally a good idea if you have a % discount.
As others have said, you take the chance there will be no club level rooms available at check in. If you are OK with this, no problem.
We have had various experiences. A few times, we have asked to pay to upgrade and gotten the same discount for the upgraded room (e.g. AP rate to AP rate, GP code to GP code) and simply paid the difference just as if we had booked ahead. A few times, we were told the difference would adjust to rack, which we declined. Many times, we've been told club is sold out, nothing available at any price!