Since you already have your visa, there is no point in changing it. You would only do so if the was a substantial, material change to the circumstances of your visit.
In this case, your situation is such that your travel itinerary has changed, even though you are still primarily traveling to Finland. You do not need to notify the embassy or ask for a visa change.
Croatia offers visa exemption for Schengen holders under these conditions:
- You must hold a multiple entry visa, valid for all Schengen states
- The visa period must cover your stay in Croatia
- You must not have used all your days in the 90/180 rule (in other words, you cannot spend 90 days in Schengen states, and then use the same visa for Croatia, you would need to exit the Schengen zone completely)
- For double-entry visas, you must have one entry remaining.
From the information you have provided, you meet all the criterion, so enjoy your stay.