I am currently an original beta customer from republic wireless and would like to keep my current plan but i am thinking of replacing my moto g 1st edition with a moto x 1st edition. Can this be done and could i keep my original plan?
Yes, and yes. Just activate the X1 as a replacement for the G1.
Yes, as long as the moto X1 is a Republic Wireless custom version of the phone.
To keep your original plan - make sure that you activate the Moto X1 as an upgrade/replacement
to your existing Moto G1.
See here for details
Do you already have that X1? If not and the intent is to acquire one, it will need to be sourced previously used via the secondary market. Republic offers guidance for that here: Buying and Selling Used Republic Wireless Phones.