I have successfully upgraded the stock 16GB Micro SD Card to a SanDisk 200GB SDXC. I could of damaged it by accident so if you are new to opening up handhelds here are a couple of tips I hope you may find helpful.
1) The back plate is clipped on as well as the 4 screws. Release the sides first then top, carefully lift up and ease the bottom so you do not damage the clips.
2) The battery fixed on to the back plate, be careful removing the back plate. If it pops suddenly it break the wires.
3) I accidentally powered on my RG350 while opening it. The screen will be face down so check you have not accidentally powered it on, then carefully disconnect battery by gently unplugging the connector.
4) I Flashed the SD Card with Win32 Image Writer, then expanded the (Ext4) partition using Disk Genius as partition wizard would not resize the partition.
5) Very carefully remove the anti tamper sticker from the stock Micro SD Card and replace with your own. Ensure your hands are not clammy or sweaty as any unnoticed moisture on the SD Card contacts could damage the card or device.
6) Reconnect the battery, then when clipping the back plate on, do it in reverse of how you removed it. Start at bottom, then clip the top, then then sides and screw back together.
I hope this helps someone.