The easiest way that has been working to get a subsidized phone using your unlimited data plan's upgrade without losing your UDP is to buy from Best Buy online. Now since Moto Maker uses the Best Buy system, at least one poster on another forum reported it worked using his upgrade on Moto Maker, too. I have not personally used this method but others on another forum have and have posted screen shots showing it worked for them to keep their UDP.
There is a glitch in the Best Buy system (and in Moto Maker since it is the same system) whereby if you use your UDP line's upgrade, it tells you that you are going to lose your UDP but it does not make you select a new plan before checking out. And when you complete the check out it says Keep Current Plan. This appears to be true for all phones except the Droid Maxx. To progress through check out with the Droid Maxx you have to select a new Share/More Everything plan. Why that is no one knows and I suspect this glitch will be fixed eventually for all phones as it has been fixed for the Droid Maxx.
You also need to ship the new subsidized phone to your home to do the activation yourself. Do not ship to the store because many stores will not give you the phone without activating it with the SIM card it shipped with in it. That is the step you want to avoid - you do NOT want to activate the new subsidized phone with the SIM card it shipped with in it. If you do, you will be switched off of your UDP and onto the 2GB data plan. Instead, you want to swap out the SIM card the new phone shipped with and replace it with your current SIM card. If they are different sizes get a SIM card cutter/adapter as needed.
All Verizon 4G LTE phones are shipping with the SIM cards already installed in them. All you have to do is turn them on and they connect to the network and this is all it may take to lose your UDP. So the first thing you should do when you get your new subsidized phone is remove the SIM card it shipped with and throw it out. Do NOT turn on your new subsidized phone before removing the SIM card it shipped with.
So here are the steps:
1) Order a subsidized phone other than the Droid Maxx from Best Buy online using the upgrade on your UDP line, choosing Keep Current Plan and selecting No Change each step of the way, and taking screen shots of the ordering process at each step to document;
2) Ignore the warning that you will lose your UDP;
3) Do NOT select a new plan; choose Keep Current Plan and select No Change at each opportunity throughout the ordering process;
4) Select ship to your home address and do NOT ship to store;
5) Before you check out confirm that it says Keep Current Plan;
6) When your new subsidized phone arrives to your home, do not turn your new subsidized phone on, remove the SIM card that shipped with the phone and throw it out, shut off your current phone and remove the SIM card from your current phone. Then put your current SIM card in your new subsidized phone - get a SIM cutter/adapter as needed if the SIM cards are different sizes;
7) Turn on your new subsidized phone - this should activate the new subsidized phone;
8) Log onto Verizon online and confirm that you still have your UDP on your line; and
9) Keep checking for the next 14 days to make sure you keep your UDP.
Again, I haven't done this myself, but I have read about others doing it on another forum and they posted screen shots. Kicking you off of your UDP appears tied to activating the new subsidized phone using the SIM card that shipped with the phone. My understanding is that this works because you aren't doing that and because the Best Buy system warns you that you will lose your UDP but does not make you select a new plan. The only exception so far is the Droid Maxx, but I am sure in time this glitch will be fixed for all the phones. But for now, from all I have read it is still working.
There has been 1 poster on another forum who claimed the Best Buy Method outlined above did not work for him and he was switched off of his UDP to the 2GB plan. However, he did not do the SIM card switch himself without involving Verizon. He said it didn't work and so he called Verizon and the phone rep did something to make it work and that is when he lost his UDP. Moral of this story - if you are going to do this do NOT involve a Verizon rep or a Best Buy rep for that matter. If it doesn't work for you for some reason, my advice is to return the phone to Best Buy and have your upgrade restored. You may or may not be able to get out of the restocking fee if the phone has never been activated. Whatever you do, do not involve a Verizon or Best Buy CSR in the activation process. Other posters have posted screen shots showing they kept their UDP when they didn't involve a Verizon rep. YMMV.
Also, other posters on the other forum that have made mistakes and not followed the protocol precisely have been able to return their phones to Best Buy, get their upgrade restored and get their UDP back as long as they did it within 14 days.
If you want a Moto X Dev Ed, which you can't get using an upgrade, I suggest you use your upgrade to buy an iPhone 5s and sell it and use the cash from the sale towards purchasing a Moto X Dev Ed. I worked out the math on that in this thread:
Using Your Upgrade to Buy an iPhone 5s to Sell So You Can Buy a Moto X Dev Ed