Yes it is important to change your impeller pump.

Two of the things on either a two stroke or four stroke outboard boat motor that require regular maintance are the impeller and thermostat. The impeller will usually be the cause of 95% of outboard engine overheating problems. It is rare that a thermostat goes bad, but it does happen.  Do yourself a favor and change the impeller/water pump every two or three year's regardless of usage. If you live around the ocean's, and operate your outboard motor in salt water, be sure to flush your motor with fresh water after each use.