You can create an image and either send it to a 2.5" drive in the system or send it to an external drive. Now if you have a 3 cell battery then you can install a 2.5" drive in the system. Alternatively you can purchase an M.2 enclosure and place the new M.2 drive in the enclosure. Then you can clone the old drive to the new one. To create an image or clone the drive use a program such as Macrium Reflect Free available here. They have a number of tutorials on their site. You can also find videos on YouTube here for a tutorial. If you clone the old drive you won't have to worry about booting from a USB drive. If you image the drive then create a Macrium Rescue Disk to boot from. F12 at boot should get you into the boot menu.  
My G3 is about a year old. It has 8gb ram, 1 m.2 256mb and the open spot for a 2.5”  sata  ssd. Adding the 2.5 should be the easy button, but I would like to replace the m.2 with a 1Tb stick to keep the speed. I would also, in the future like to dual boot Linux too from the m.2.  I understand that I would need to clone the 256 on to the 1Tb with 3rd party software and there can be problems booting from usb.  I’m looking for input from people that know more than me.  Thanks Markinwi
G3 3500 ssd is full