Slow down Stwicky!
Although your plan offers a Roth 401k and in-plan conversions, going that path may have significant irreversible side effects that are not obvious up front.
Since you have what appears to be a traditional 401k with significant after-tax contributions, you may still have some very attractive options available to accomplish your goal.
Does your plan allow partial withdrawals-distributions from the 401k before age 59 1/2 while you are still employed? If so great.
What you probably will need to do is to setup a series of transactions where the money ends up in a traditional
IRA (ie the taxable porition) and a Roth
IRA (the non-taxable portion). Note that a Roth IRA is a very different animal than a Roth 401k.
Back shortly after 2010 when the rules changes for Roth IRAs (with regards to income), I did a series of transactions from the 401k to the IRAs and also from the IRAs to the 401k to "isolate my basis". You can read up on this more at fairmark.com. They are experts in these strategies over there.
When I started doing this, it always required Indirect 60-day rollover(s) from the 401ks to the IRAs. I had to front the money for any tax withholding.
Since then, the regulations have changed and allowed for direction rollovers to accomplish the task. I have confirmed this recently in it was much easier.
My advice to you is
#1) Don't try to do this too quickly -- it takes research and understanding/following the rules
#2) study about this with resources such as fairmark.com
#3) Study the IRS "model language" (ie 402(f) notice) that employer plans are required to offer before a plan distribution. See a publicly available
version here.
#4) Understand your history -- Be sure to understand why 60-day rollovers were not allowed prior to a few years ago. This will give you better context to understand if you can take advantage of the new rules that allow direct 60-day rollovers to accomplish this strategy.
#5) when you think you understand the process, do a test with only a small amount - say $2,000. Then wait until you do your taxes the following year to confirm that you understood everything properly - ie no tax bill and your test money in your IRAs (traditional and Roth)
#6) Understand that laws can/do change. Take any recommendations you see on a web site with a grain of salt. Verify that the technique is still valid by verifying with current law/regulations and your own plan SPD.
In my case, the research paid off and I have been able to move $100,000's to our Roth IRAs (our employer plans allowed after-tax contributions to our traditional 401ks up to a total of ~ $50,000 each every year). I have since ER'd and DW is soon to follow.
Just realize that this is a benefit to those who can read/understand the rules and carefully take advantage of them.
-gauss