I did exactly this a few months ago. New 120g where my 50g was. I can give you a few ideas on what to do and what not to do...Take them all with a grain of salt though.
First of all, you are very, very smart to want to take your time with the new tank. For all the effort and $ you are likely to spend on the new one, you probably want it to be perfect to start, especially when it comes to aquascape. As it was, I took my time with mine, but in hindsight, would like to have taken longer, especially with my OCD for things being perfect and in straight lines. Problem is the livestock who were kinda dangling in the temporary smaller tank I had set up elsewhere to hold them.
My advice, in brief, is this. Pretend you're moving. Move out the live stock - fish corals etc - into temporary buckets and bins. Then move your existing set up to someplace, hopefully nearby, that's out of the way of the new tank. Then put everyone back in the old tank, sump, skimmer, lights...the whole kit and kaboodle. With that back to close to the way it was, you'll have a stable situation with your livestock that you can then take your time setting up the new system. You can take weeks or even months now to get the new system going without having to worry about the health of your critters because really all you did was move their old house to a different spot. Yeah you have to put them in buckets and junk for the interim, but that's unavoidable.
I didn't use this approach. I set up a temporary 29g and threw all my fish and coral covered rock in a temporary tank i put on the floor. The rest of the rock went in buckets with heaters and power heads. I lost 3 fish and a handful of corals. I didn't run all my lights, no skimmer... etc. At least one of my fish died due to some bruising she got from either the lack of moonlights in the temp tank, or being netted to get her out of the original one. Which brings me to...
Some other tips...
Avoid netting your fish if at all possible. They hate it. Especially tangs, if you have any. Coral them with your hands into one of those tupperware type dealies. This mostly implies removing your rock and such first, but that's unavoidable when it comes to catching fish. YOU WILL NEED A FRIEND to help with this.
Make a lot of water. By a lot I mean however much you think you need when you started reading this? Double it.
Speaking of doubling things, triple the amount of time you think you might need to move everyone. Better to over-prepare than under prepare.
At least 8 rolls of paper towels. Extra buckets, extra heaters, extra power heads. Borrow from a friend. Oh and a mop. The floor is gonna get wet. Very wet.
If you do the above and essentially move the existing tank to a new spot for a while (also assuming you have room in your home to do that) you will not get the aquascape exactly the way it was before you started. Don't obsess over this, just get it so that the corals are getting close to the right amount of light / flow and the fish have room to swim and hide.
This one you're not going to like - prepare yourself for the possibility of losing some livestock. Moving creatures is stressful. It might happen, despite all the precautions in the world. Hopefully it doesn't.
Bribe a friend or friends to help. Beer and free food go a long way
Good luck, and have fun. It was rough when I did it. I had a friend helping me and it wasn't THAT bad really, that one Saturday, but the critical mistake I made was in staying in that smaller, temp tank with minimal equipment support for as long as I did (it wound up being 3 weeks). In hindsight I should have just moved everyone out, moved the 50 and stand and sump etc, then put everyone back in. Then I could have taken my time with the new set up and did a few things differently and probably would not have lost a few of the losses I had. Also, there's likely to be a delay with the new tank. Deliveries take time. Work/life/wife/kids/sleep get in the way. You might think you can plumb and whatever the new tank in a day... I did. It took a week and a half.
Most importantly - have fun. Think about how it's going to be so awesome when you're done and all that extra room you're going to have for more goodies
Post pictures and updates, lots of good people here will offer plenty of advice.
:lobster: