It's a grey area of cost / benefit.
The costs of upgrading engines or buying new ones are going to be very expensive for a couple of reasons.
The first is buy/ lease back financing. These costs will be about 0.80-0.82% per month of the list price of about $10 mill which has to eat into margins, even with the fuel and maintenance savings.
The second is, that an operator will have to do a lot of "wheeling and dealing" to ensure that the list price will stay affordable as these high thrust engines are also in high demand.
Upgrading the modules may be the answer assuming there's still good life left, but depending on the version, an upgrade can still cost up to $4 mill.
Buy or upgrade? Lots of homework for the bean counters I'd say, and CN may not have the credentials for this task.