So the gas company is providing 30 psig and you are looking to change the downstream system from 15" w.c. to a lower or higher pressure.
If you are desinging (redesigning) a natural gas pipe system where you are taking the gas company pressure down to the building pressure, you will need to consult the applicable gas company requirements for sizing piping and/or the appropriate fuel gas code (whether it is the NFGC or the IFGC). They have tables and/or equations to use.
You will also have to look at changing the regulator or at least the spring.
Using the Spitzglass equation for low pressure (<1.5 psig) or the higher pressure equation (for >1.5 psig) are generally useful.
To answer the density question, natural gas is compressible, so the density will change with change in pressure. Generally, if the pressure drop is less than 10%, you can use either the density of the upstream gas or the downstream gas. If the pressure drop is 10%-40%, the general rule of thumb is to take the average of the two densities. If the pressure drop exceeds 40%, you have to use empirical equations which are quite fun to use.
Going from 15" w.c. to 7" w.c. would likely require larger pipe. I would suggest using 2 or 5 psig inside and installing additional regulators at the equipment if there are not too many of them and if the regulator vent pipes can be run easily.