It depends on what you mean by consistency. Possible ideas include the following :-

• Completely reversible - you can get out what you put in
• All exisitng queries give exactly the same answer with the new structure
• The two data structures are formally equivalent to the old one in the sense that all true (and false) statements about the original two files remain true (and false) for the new file

This is tricky to say the least, and requires a lot of consideration. My advice would be simple, unless you have to do this, don't!