First, you could create a business rule to synchronize the values of Environment with the values of Used For, and this could technically be a simple business rule, but your values would have to be identical and I believe out of the box they are not identical. Thus you would need to either make it so the values are identical in both fields, or (preferably) script the values on each condition (note, you'd need to update this script if you add or change what values are available).
That said, my recommendation would be to consider this as a one-off data migration and data model change instead, moving all of the data in a big bang to Environment, enabling Environment and removing Used For from the User Interface, and just considering it a new attribute that you need to populate. There would be some short term change management involved, since your users may be populating and reporting the Used For field today, but it might be a better choice to take the hit all at once rather than prolonging the use of the Used For field, which is now considered deprecated.
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