Implications of changing fields to CASCADING SELECT fields

[Deleted user]

I have 2 drop down fields currently - both are SELECT LIST (single choice) type. 

I would like to request to have both the drop down fields converted to CASCADING SELECT LIST so that the list in the second depends on value in the first. 

Also, I was going to change my values in both these lists involved in the Cascade.

Question is : I have over 2000 Jira Tickets for which the values as they exist currently are present. I would like to keep it that way for past tickets.   If I request the above field type and data values change, will I lose all the old values that I have on these 2000 historical tickets (since those values now don't belong to the drop-downs anymore) ?