Ahoy @User_EU-1e81e890 ,
I had a look at your design and the product. The problem is not that you want to change the print area from back to front, but that you want to make the design smaller. It is a vector design. Your desired product can only be printed with flexprint, which is why a vector file is necessary. However, there are certain criteria that have to be met for flexprint. For example, the smallest elements of a vector graphic must not be smaller than 1.5 millimetres.
So if you reduce the size of your vector design, it cannot be printed - the same would happen if you reduced the size of the design on the print area at the back.
So if you want to print a logo for the print area on the front of the chest, you have to create it in the right size so that the smallest elements are not smaller than 1.5 millimetres. Then you upload this vector file and can place it on the chest area. However, it is no longer possible to reduce the size with this design.
I hope this helps you.