What to do on existing data when there’s a lookup change?

I'm working on an existing system, with existing data.

Clients want to update some lookup data. I checked and since the existing data lookups to the "old" value, how are they handled?

Here's an example:

Old scenario: when an item is created, the default lookup value of Field1 is 5,000.New scenario: when an item is created, the default lookup value of Field1 is now 10,000.

Now, I'm struggling to find the right approach for their existing data. Should I:

1. Set existing data as out of scope (meaning I will not update existing data) so that it will make my life easier and the completion faster?
2. Update existing data on the backend so they will lookup to the "latest" data?

I know the next thing to do is to ask clients what they want to do on the existing entries. But do you guys have any suggestions so this development will run as smooth as possible? The scenario above is just a sample, the actual logic is actually pretty complex for it to lookup.

Happy holidays btw! I know it's holiday season but this thing is bugging my mind haha.