No wasting of credits. The only time you use a vin license (credit in hp tuners terms) is when you flash a file into a pcm for the first time with that cable. Once licensed to that pcm, you can make as many operating system changes (standard or custom) as you like with no further costs.
The quickest way to get up & running with a custom OS is.
Find the custom operating system you wish to use at C:\Documents\My Documents\EFILive\V7.5\Bins\Custom OS.
Do a full flash to your pcm of the custom os. (here is where you will use your vin license)
Open the stock tune you are using & calibration flash only. This will put your stock settings onto the custom os.
Read the file from the pcm & ensure that the custom settings (A00x) are at sane values as described in the tutorial.
Go to the B3647 fuel mixture table & correct this due to the axis change.
Save & then cal only flash this as your base tune.