The short answer is no-- over time, your beer will drop down to the carbonation level which you would get at 5psi (at serving temp).
My question back to you is, why do you want to serve at 5psi? It seems to me that you are looking to serve at 12psi.
I will venture a guess and say that currently your system is unbalanced, and thus at 12psi, the beer comes out of the tap too fast. In order to correct this, you don't want to serve at 5psi; rather, you want to add enough resistance into the system so that 12psi pours at an acceptable rate. The easiest way to do this is to increase the length of your beer lines. There are calculations availible (which I don't have readily at hand) which will tell you the length of tubing you need to maintain 12psi.