The two chains are so different I don't think you could simply port a program over. The embeds are very different and your existing embedded code won't be using the ABC classes, so what you end up with is a mix of both types which will be quite hard to manage in future upgrades.You could use the Windows layouts again but almost all the rest will probably need replacing.
However most programs benefit from a review and tidy-up of the design after several years and although it looks daunting you often end up with a much slimmer and efficient product. If you take this opportunity to redesign with all the experience of previous use the functionality can be much improved. Only the ABC chain is being developed so staying with legacy deprives you of new things. It all depends on if you have plans for lots of new development. If you do it is worth biting the bullet and making a new ABC compliant program. If not stay with the legacy until the product is retired.