What are the tax implications of switching from W-2 to K-1 during the same year?

There are many issues here and probably too many to detail, but I will provide an overview:

1. There will not be any double tax impact.  I am sure that your partnership agreement is that upon admission or exit, the allocation is per day per unit.
2. As a partner, you are now self-employed.  This means that you will need to make quarterly estimated tax payments.
3. I would also assume that you will be provided with either a guaranteed payment or distributions to cover your tax impact.  If you are receiving a guaranteed payment, this is the downfall for many individuals.  They forget that no taxes are taken out and that this guaranteed payment is subject to self-employment tax AND income tax.  Set aside $$ so you are not caught short.
4. Additionally, as a partner, probably most fringe benefits that you are now receiving tax-free will no longer be tax-free.  These will be subject to tax.  Discuss this with someone at the Company so you understand the impact to you here.
5. The IRS has a safe harbor for estimated tax purposes.  I have attached a link below to the 2017 form 2210 Underpayment Penalty form.  I would recommend that for your 4th quarter estimated tax payment you pay in enough to cover a safe harbor; either the 100% or 110% if you are in that AGI bracket.
6. Finally, it is VERY important that you maintain a basis schedule of your interest in the partnership.  If your K-1 section L is marked "tax basis" then you should be good to go.  Anything else you need to maintain the basis schedule.  A link to the 2017 form 1065 K-1 is below.