Is it possible to modify field types in a "live" project?

I asked this question in another thread but think it got buried. One of our PMs created the project  that this sort is for and he created a bunch of the fields as type  "state[1]".

Can  I change the type of a field (a bunch of fields actually) from state to  enum for a in use project?  We have a bunch of records already w/ lots  of comments, etc.  I see that I can't do it thru the UI but maybe  there's another way?

Thanks in advance for your help,

Jerry