It's really not clear what you're asking. Are you asking about converting your 4-yr to a GRFD or are you saying you're a cadet currently on a 4-year scholarship, but you prefer to drop the scholarship and attempt to become an SMP cadet instead? Whatever for? If you desire to go Guard/Reserve upon commissioning, just select that option at the appropriate time.
The only time you can convert to a GRFD is between the beginning of your MSII year and Sept of your MSIII year. If you're in that window, and there are available scholarship allocations, and you're 100% positive you do not want to go active, amd you're not nursing or G2G, then look into it. You'll need to fill out a CC Form 226-R. Talk to your Cadre.
If you're talking about dropping the scholarship altogether, then unless you accept National Guard tuition assistance, you will by no means receive the same benefits (I.e. money) that you get from your scholarship. In order to receive benefits, you will most likely be required to attend Basic and AIT, delaying your schooling and therefore commissioning by up to a year.
Can you give us a little more info as to your thought process, because nothing about this screams good idea to me.