12 Volt DC wiring in existing home for under soffit LED lights
The main issue you'll have is with the power supply.  The "old" landscape lighting power supply is designed to drive incandescent bulbs.  Incandescents don't care whether power is AC or DC*.  

The problem is, LEDs very much do.  Now, you know 120VAC LEDs don't care; that's true, but the way they fix that is with a diode bridge rectifier. The rectifier has a voltage drop across it of about 2 volts regardless of input voltage - no big deal on 120V, but a big deal on 12V.   So 12V lighting usually omits the rectifier and expects you to polarize correctly.  

Best case: the LED shimmers, being on 50% of the time. Worst case, the reverse voltage damages the LED, or the fact that LEDs are non-linar does not play well with that 16.9V RMS* peaks, and fries the LED. 

I recommend one of two options: 


Get a correct power supply for this application.  I for one am a fan of GE power supplies.  However if you prefer this Amazon/eBay "fell off a truck in Shenzhen" cheap Cheese, get units that are much larger than your actual draw.  These products are wildly underbuilt and overrated, and will fail quickly at anywhere near nameplate. 
Or keep your power supply and deal with a technically competent domestic supplier who really knows his product and can address the "working on AC" issue. I'm sure there are products that do; they aren't sold for peanuts on Amazon/eBay/Ali*. 




While you're at it, you might want to revisit whether you want these bulbs to be all one color (and what color temperature, also CRI).  Or whether you want them to be color-changeable in unison; or be individually addressible - each color set separately.  This can't be retrofitted; each of these schemes requires different initial wiring.  

A monochrome system requires only 2 wires (earth/ground is not needed for low voltage; DO NOT USE the AC power system's earth/ground as a current return!) An RGB "in unison" system requires 4 power wires (5 for RGBW), with 1 wire being larger than the others.   Individually addressable systems vary by the system, but only 2 wires handle all the power, the others are signal. 



Sizing the wires: 

You may need to do some light conversion between watts and amps, as needed.  Amps x 12 = watts, close enough. 

Once you know how many amps you will be carrying what distance, head off to a voltage drop calculator (I like this one) and punch in your numbers.  Voltage is 12.  Percent allowable, I'd put in 10%, or 99% if you just want to see what the voltage drop will be.  Don't use 3%.  For current, put in your actual draw current of your lamps.**

High voltage drop is "dangerous" to you in a different way.  Now, overheating the wire is not an issue with this voltage drop calculator -- it automatically upsizes the wire to prevent overheating.  The problems are


12V systems flow more current, to make up for the lower voltage. More current equals more voltage drop. 
12V systems don't have much voltage drop to spare.  A 7V drop isn't a big deal on a 240V circuit, but it dooms a 12V circuit. 
LEDs are non-linear.  With an incandescent bulb, dropping voltage 10% drops current 10%, resulting in a 19% reduction in light - mind you, that's linear.  LEDs are much worse.  


The upshot is if you don't keep a close eye on your voltage drop, your faraway LEDs will be noticeably dimmer than the nearer ones. 

I would draw it out on paper, then calculate each segment of voltage drop separately.  You may need to allow some voltage drop on a long run, and focus on keeping each of the bulbs relatively equal. 

A little bit of sharp-pencil work will help your project go much better. 



* in fact, cities were already being wired for 110 volt DC when they converted to AC power.  (110 was due to two "bumps" from the original, nice round 100V). They needed to find an AC power voltage that would just work with all those existing 110VDC bulbs. (the main purpose of electricity in those days was lighting.) They figured out the bulbs were happy with AC voltage that was 155 volts peak to center, and this is how they worked out the "root mean square" method of defining AC voltage.  Since then, in the US/Canada there have been 2-3 more bumps to 120V, our now-official voltage. 

** Some people will get mad at this advice, because they have been taught to always use 3% for allowable drop and the circuit breaker capacity (i.e. 20A) for the current. That's not always a good fit; this here is an example.