Impact on modifying deployed table data.

I do not understand what your actual question is. Please explain your ask in more detail.

You can make the changes but the things to keep in mind to scale efficiently is Active process instances will not have those 2 new fields. So ensure if any precedent rule of that process model should not reference those fields directly. Have default values setup in case the field can't be indexed for such cases.

Any rule or process you are modifying referencing these new columns, keep in mind not all instances will have those fields so don't use dot operators, user property() functions to reference those to avoid the ' cannot index field error ' in production 

Adding new columns to an existing production database table has no impact on objects currently using that table - all existing processes, interfaces, and reports continue working normally since they only reference their original columns. The new columns will remain empty until you explicitly update objects to populate them.

To handle this safely, make the new columns nullable, update your record type, and deploy through standard(Dev-Test-Prod) environments. You can then gradually update objects to use the new columns as needed - this is low-risk since you’re adding, not modifying existing structure.​​​​​​​​​​​​​​​​