Changing Product Lines and Keeping History

We want to clean up a bunch of product lines by moving items into new ones, but we want to see all the history in the new product lines. We're reducing the product lines from about ten to four.

Is this the correct way of doing this?

1. Create new item numbers in the new product line, e.g. Create a new item 456 in a new product line B, then merge current item 123 in current product line A into it.

2. Once that is done, re-merge item 456 in product line B into item 123 in product line B. Or, do we just have to renumber 456 as 123?

Thanks!