I have a requirement where I need to add few columns to a Data Extension on which a journey is active and running at present with real customer data. I added few columns on a test DE and checked the attribute group set in Data Design, without any un-mapping and mapping, it showed the newly added column. So, kindly help me understand the consequences of doing this to my journey and also on the contacts that will meanwhile enter the journey when we are updating the DE.
The customers already in your journey shouldn't be affected, however they won't ever have the values you subsequently add to the new fields as the point of time when they entered the journey the fields didn't exist. This could of course impact any decision splits you make along your journey, or if you intend on using the field values in a later email.
You are best to pause any SQL automations for the journey entry whilst you make the change, otherwise contacts could be processed whilst the change.