Others have mentioned re other titles, LYNX's 'stretching' (to the absolute limit) of material which already exists. Basically they're taking existing plates and text and just replicating them across several titles so they'll hope to sell at least two titles here, that we're aware of, rather than one.
Have you looked at the guides? Whilst many of the original plates are being used, a lot of new illustrations have been / are being commissioned. The text is new and purpose written. HBW text was oriented more towards use at home or in a museum, which is appropriate for a handbook. You will find that Lynx and BirdLife International Field Guides text is written for field use. Yes, of course there will be some degree of duplication between countries with very similar avifaunas, as there is with other field guides series. However, both text and plates will reflect specific national identification challenges, racial variation, vocal variation, etc. New range maps and distributional revision are another key strength.
Best wishes, Chris