Ah yes, that can be confusing. I went through the same thing when helping someone set up an S-Corp a few years back. The "business" opened in say, 2010. But the "Parnership" was established in 2016. Doesn't change the business open date. It only changes your ownership percentage, and the acquisition of a new partner on the date the Sole-proprietorship was no longer "sole".
Things "might" be different since this is not converting to a corporation. So I"d say if the 2016 open date for the partnership will still allow you to import the "prior depreciation already taken" on your assets, then you're fine. But I don't think it will because you can't put an asset in service in a business, before it's open for business. At least, I discovered that to be true with an S-Corp.
