It's very tempting to upgrade to 1.3.9 but due to the large number of customizations in my particular instance, a simple WinMerge process will not work for me. That is, I need to start from scratch and re-apply and test each and every add-on and code customization that's currently on my 1.3.8a operational site. Which means that, for lack of such time and resources, I will have to wait for 2.0 (to go through this HUGE effort only once instead of twice).

Still, I would like to share with you an excellent article that I found in my search for good reasons to upgrade to 1.3.9:

pro-webs.net/blog/2010/05/06/zen-cart-1-3-9-whats-inside/

Through that article I found a thread listing all mods/addons that are compatible with 1.3.9. Unfortunately, most of the addons I am using are not listed there.

If you have a technique that would allow me to minimize the time needed to migrate my operational store from 1.3.8a to 1.3.9f, I would be happy to reconsider.