It is much better PLAN to get brand new ASW ships than to modify older ones, meant for other tactical role... Modifications might take more time than building new ones, at the other hand, modifications for CCG are simple and it will goes much faster, so i see this as a win win decision.
Turning a Type-056 into a Type-056A variant with ASW capability should mean
1. Adding a Variable Depth Sonar
2. Adding a Towed Array Sonar
3. Adding some electronics to manage this
4. Potentially adding a sonar dome?
This would take far less time than building new Type-056A corvettes