Hi Ash, 

As part of your tasks carried out so far, have you mapped what data will go where? For example, the 'Client Address' field in Legacy system will map to 'Primary Client Address' field in New system.

This data mapping will be crucial to the success of your task. Developers etc will need to know where all the data goes. You could write a User Story for each and every field, and without knowing how many of them there are its a little hard to advise, but I would complete a table in Excel or something similar. Have one column which states what the Legacy field is called, another stating what its format is (is it alpha characters only for example), the next column says what the maximum character length is in legacy system. The do the same column, but for the new system.

You will need to make sure that the new system fields at least match the old fields, otherwise yo wont be able to move the data over (you'll get errors all over the place). You may need to speak to stakeholders to find out how long they want fields and what sort of data is to be entered into the fields (just alpha, alpha & Numeric etc.).

Then, once that exercise is complete I would speak to the developer who is doing the data migration, and see how he/she wants to split the User Stories out. Do they want just one User Story, or multiple? Create the User Stories as advised by your Developer and make sure you attach the Mapping table to it, so that the Developer can relate to it.