If you adjust the parameters, only newly created password hashes are affected. Existing password hashes use the parameters that they were created with.
To avoid having the system run out of memory or the CPU being overloaded, tune the settings for your hardware environment before going productive. For example, if the system has a lot of RAM but few CPU cores, you can increase the memory requirement while reducing iterations. OWASP provides several examples of settings that offer equivalent security while having different CPU- and memory-requirements.
Even if most of the users in your Identity Manager service log in via other methods than username and password (for example, SAML or certificates), there can be internal users with username and password that may be used by external services to log into Identity Manager, for example, for callbacks.