How to downgrade Magento 2.1 to magento 1.9?
How to do that automatically instead of obvious export/import path?
Looking for an expert opinion, please no 'why do you need that?' questions.
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Like @Nikola said, there are methods to upgrade from 1.9 to 2.1, but there are no options that I know of to downgrade. So my best guess is you have to write this yourself.
Now as for the database structure of 2.1, a lot of it is (more or less) identical to the database structure of 1.9, but not everything. So I can imagine you could write a downgrader of some sort with much trial and error. However, I don't think it can be done for the full 100%, but you could have luck with EAV entities, products, categories, cms pages/blocks, customers and addresses. I think you will run into more problems with orders and invoices and stuff, but the biggest problem you'll face will be the relations. Needless to say: it will not be easy and I suggest to create such downgrader step-by-step and write unit/integration/end-to-end tests for it while you're at it. Because even the slightest error could result into a corrupt database where you don't know where to start looking for answers.
If you're only concerned about migrating products and customers, I'd suggest looking at the default ways of exporting data out of 2.1 and importing it into 1.9. It could be that you're faster done by just copy/pasting exporting/importing everything than trying to automate it.
As for templates: the template structure of Magento 2 is completely different than what you're used in Magento 1, so you'll have to re-create the template for Magento 1.
As for extensions: you're out of luck there. No extension that works on Magento 2 will work on Magento 1. So you'll have to buy the v1.9.x-version of it (if it exists), or look for an equivalent there.
So yeah... it's not an easy task and you're probably faster of creating the whole thing from scratch and try to reuse the stuff that can be re-used.