Merging a local developed stall with a live site

Still on 135sp3

Over a year ago I was working on a general improvement to my site and doing it on a local install since it was so much faster.

This was not modding just categories etc.

For a number of reasons I had to abandon implementing the change.

I have not sold that much in the mean time but am wondering whether to try and make the changes that I originally intended.

How might I go about getting details of customers and sales from the live site merged into the off site one since it would be a shame to lose this data.

Is this a pretty easy task or pretty technical?